View source for Railway electrification

{{Short description|Conversion of railways to use electricity for propulsion}}
{{Use dmy dates|date=March 2021}}
{{Multiple image
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| image1 = Catenaire 25 kv Caen Paris.JPG
| caption1 = The [[Mantes-la-Jolie–Cherbourg railway]] in France connects [[Grand Paris]] and [[Normandy]], and is electrified using [[overhead lines]] at [[25 kV AC|{{nowrap|25 kV AC 50 Hz}}]].
| image2 = Paddock Wood Station - geograph.org.uk - 3292174.jpg
| caption2 = The [[South Eastern Main Line]] in England connects the [[London metropolitan area]] with the [[Strait of Dover]], and is electrified using a [[third rail]] at {{nowrap|750 [[Volt|V]]}} [[Direct Current|DC]].
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'''Railway electrification''' is the use of [[electric power]] for the propulsion of [[rail transport]]. Electric railways use either [[electric locomotive]]s (hauling passengers or [[Rail freight transport|freight]] in separate cars), [[electric multiple unit]]s ([[Passenger car (rail)|passenger cars]] with their own motors) or both.
Electricity is typically generated in large and relatively efficient [[electric power station|generating stations]], transmitted to the railway network and distributed to the trains. Some electric railways have their own dedicated generating stations and [[electric transmission line|transmission lines]], but most purchase power from an [[electric utility]]. The railway usually provides its own distribution lines, switches, and [[transformer]]s.

Power is supplied to moving trains with a (nearly) continuous [[electrical conductor|conductor]] running along the track that usually takes one of two forms: an [[overhead line]], suspended from poles or towers along the track or from structure or tunnel ceilings and contacted by a [[Pantograph (transport)|pantograph]], or a [[third rail]] mounted at track level and contacted by a sliding "[[pickup shoe]]". Both overhead wire and third-rail systems usually use the running rails as the return conductor, but some systems use a separate fourth rail for this purpose.

In comparison to the principal alternative, the [[Diesel locomotive|diesel engine]], electric railways offer substantially better [[Efficient energy use|energy efficiency]], lower [[Exhaust gas|emissions]], and lower operating costs. Electric locomotives are also usually quieter, more powerful, and more responsive and reliable than diesel. They have no local emissions, an important advantage in tunnels and urban areas. Some electric traction systems provide [[regenerative brake|regenerative braking]] that turns the train's [[kinetic energy]] back into electricity and returns it to the supply system to be used by other trains or the general utility grid. While diesel locomotives burn petroleum products, electricity can be generated from diverse sources, including [[renewable energy]].<ref name=PMKB72>P. M. Kalla-Bishop, ''Future Railways and Guided Transport'', IPC Transport Press Ltd. 1972, pp. 8-33</ref> Historically, concerns of resource independence have played a role in the decision to electrify railway lines. The landlocked [[Switzerland|Swiss confederation]] which almost completely lacks oil or coal deposits but has plentiful [[hydropower]] electrified [[Rail transport in Switzerland|its network]] in part in reaction to supply issues during both World Wars.<ref>{{Cite web|url=https://www.swissinfo.ch/eng/a-train-ride-through-history/3729832|title=A train ride through history|website=SWI swissinfo.ch}}</ref><ref>{{Cite web|url=https://houseofswitzerland.org/swissstories/history/nation-railway-enthusiasts-history-swiss-railways|title=A nation of railway enthusiasts: a history of the Swiss railways|website=House of Switzerland}}</ref>

Disadvantages of electric traction include: high [[capital cost]]s that may be uneconomic on lightly trafficked routes, a relative lack of flexibility (since electric trains need third rails or overhead wires), and a vulnerability to power interruptions.<ref name=PMKB72/> [[Electro-diesel locomotive]]s and [[electro-diesel multiple unit]]s mitigate these problems somewhat as they are capable of running on diesel power during an outage or on non-electrified routes.

Different regions may use different supply voltages and frequencies, complicating through service and requiring greater complexity of locomotive power. There used to be a historical concern for [[double-stack rail transport]] regarding clearances with [[overhead line]]s<ref name=PMKB72/> but it is no longer universally true {{as of|2022|lc=on}}, with both [[Indian Railways]]<ref>{{Cite web|url=https://www.financialexpress.com/infrastructure/railways/indian-railways-sets-new-benchmark-runs-1st-double-stack-container-train-in-high-rise-ohe-electrified-sections/1989643/|title=Indian Railways sets new benchmark! Runs 1st Double-stack container train in high rise OHE electrified sections|date=12 June 2020}}</ref> and [[China Railway]]<ref>{{Cite web|title=非人狂想屋 {{!}} 你的火车发源地 » HXD1B牵引双层集装箱列车|url=http://www.trainnets.com/archives/34518|access-date=2020-07-01|language=zh-CN}}</ref><ref>{{Cite web|title=Spotlight on double-stack container movement|url=https://www.thehindubusinessline.com/todays-paper/Spotlight-on-double-stack-container-movement/article20178424.ece|access-date=2020-07-01|website=@businessline|date=14 October 2007|language=en}}</ref><ref>{{Cite web|title=Aerodynamic Effects Caused by Trains Entering Tunnels|url=https://www.researchgate.net/publication/245307012|access-date=2020-07-01|website=ResearchGate|language=en}}</ref> regularly operating electric double-stack cargo trains under overhead lines.

Railway electrification has constantly increased in the past decades, and as of 2022, electrified tracks account for nearly one-third of total tracks globally.<ref>{{Cite web |last=pamela |date=2021-02-19 |title=Railway electrification is expected to grow worldwide |url=https://www.railwaypro.com/wp/worldwide-rail-electrification-remains-at-high-volume/ |access-date=2023-02-17 |website=Railway PRO |language=en-US}}</ref><ref name="auto">{{cite web|title=Railway Handbook 2015|url=http://uic.org/IMG/pdf/iea-uic_2015-2.pdf|publisher=International Energy Agency|page=18|access-date=4 August 2017}}</ref>

== History ==
Railway electrification is the development of powering trains and locomotives using electricity instead of [[Diesel fuel|diesel]] or [[steam power]]. The history of railway electrification dates back to the late 19th century when the first electric tramways were introduced in cities like [[Berlin]], [[London]], and [[New York City]].

In 1881, the first permanent railway electrification in the world was the [[Gross-Lichterfelde Tramway]] in [[Berlin]], Germany.<ref>{{cite web|url=https://www.siemens.com/global/en/company/about/history/stories/first-electric-streetcar.html|
title=A detour to success: The world's first electric streetcar|website=Siemens Global|access-date=2024-08-02}}</ref> [[Overhead line]] electrification was first applied successfully by [[Frank J. Sprague|Frank Sprague]] in [[Richmond, Virginia]] in 1887-1888, and led to the electrification of hundreds of additional street railway systems by the early 1890s.<ref>{{Cite encyclopedia |entry-url=https://www.encyclopedia.com/people/science-and-technology/electrical-engineering-biographies/frank-j-sprague|entry=Frank J. Sprague |encyclopedia=Encyclopedia.com |access-date=2024-08-02}}</ref><ref>{{cite web|url=https://lemelson.mit.edu/resources/frank-sprague|title=Frank Sprague|website=Lemelson MIT|access-date=2024-08-02}}</ref> The first electrification of a mainline railway was the Baltimore and Ohio Railroad's [[Baltimore Belt Line]] in the United States in 1895–96.

The early electrification of railways used direct current (DC) power systems, which were limited in terms of the distance they could transmit power. However, in the early 20th century, alternating current (AC) power systems were developed, which allowed for more efficient power transmission over longer distances.

In the 1920s and 1930s, many countries worldwide began to electrify their railways. In Europe, [[Switzerland]], [[Sweden]], [[France]], and [[Italy]] were among the early adopters of railway electrification. In the [[United States]], the [[New York, New Haven and Hartford Railroad]] was one of the first major railways to be electrified.

Railway electrification continued to expand throughout the 20th century, with technological improvements and the development of high-speed trains and [[commuter train|commuter]]s. Today, many countries have extensive electrified railway networks with {{val|375,000|u=km}} of standard lines in the world, including [[China]], [[India]], [[Japan]], [[France]], [[Germany]], and the [[United Kingdom]]. Electrification is seen as a more sustainable and environmentally friendly alternative to diesel or steam power and is an important part of many countries' transportation infrastructure.

== Classification ==
[[File:Europe rail electrification en.svg|thumb|Electrification systems in Europe:
{{legend|#c0c1c5|Non-electrified}}
{{legend|#003399|750{{nbsp}}V DC}}
{{legend|#5bc6ff|1.5{{nbsp}}kV DC}}
{{legend|#46a43b|3{{nbsp}}kV DC}}
{{legend|#c39467|[[15 kV AC]]}}
{{legend|#ffd617|[[25 kV AC]]}}

High speed lines in France, Spain, Italy, United Kingdom, the Netherlands, Belgium and Turkey operate under 25{{nbsp}}kV, as do high power lines in the former Soviet Union as well.]]

Electrification systems are classified by three main parameters:
* [[Voltage]]
* [[Electric current|Current]]
** [[Direct current]] (DC)
** [[Alternating current]] (AC)
*** [[Frequency]]
* Contact system
** [[Overhead line]]s (catenary)
** [[Third rail]]
** [[Fourth rail]]
** [[Ground-level power supply]]

Selection of an electrification system is based on economics of energy supply, maintenance, and capital cost compared to the revenue obtained for freight and passenger traffic. Different systems are used for urban and intercity areas; some [[electric locomotive]]s can switch to different supply [[voltage]]s to allow flexibility in operation.

=== {{Anchor|Voltage}}Standardised voltages ===
Six of the most commonly used voltages have been selected for European and international standardisation. Some of these are independent of the contact system used, so that, for example, 750{{nbsp}}V{{nbsp}}DC may be used with either third rail or overhead lines.

There are many other voltage systems used for railway electrification systems around the world, and the [[list of railway electrification systems]] covers both standard voltage and non-standard voltage systems.

The permissible range of voltages allowed for the standardised voltages is as stated in standards BS{{nbsp}}EN{{nbsp}}50163<ref>EN 50163: ''Railway applications. Supply voltages of traction systems'' (2007)</ref> and IEC{{nbsp}}60850.<ref>IEC 60850: ''Railway applications – Supply voltages of traction systems'', 3rd edition (2007)</ref> These take into account the number of trains drawing current and their distance from the substation.

{| class="wikitable"
! rowspan="2" | Electrification system
! colspan="5" | Voltage
|-
! width="80" | Minimum<br>temporary
! width="80" | Minimum<br>permanent
! width="80" | Nominal
! width="80" | Maximum<br>permanent
! width="80" | Maximum<br>temporary
|-
| 600{{nbsp}}V [[Direct current|DC]]
| 400{{nbsp}}V
| 400{{nbsp}}V
| 600{{nbsp}}V
| 720{{nbsp}}V
| 800{{nbsp}}V
|-
| 750{{nbsp}}V DC
| 500{{nbsp}}V
| 500{{nbsp}}V
| 750{{nbsp}}V
| 900{{nbsp}}V
| 1,000{{nbsp}}V
|-
| 1,500{{nbsp}}V DC
| 1,000{{nbsp}}V
| 1,000{{nbsp}}V
| 1,500{{nbsp}}V
| 1,800{{nbsp}}V
| 1,950{{nbsp}}V
|-
| 3{{nbsp}}kV DC
| 2{{nbsp}}kV
| 2{{nbsp}}kV
| 3{{nbsp}}kV
| 3.6{{nbsp}}kV
| 3.9{{nbsp}}kV
|-
| [[15 kV AC railway electrification|15{{nbsp}}kV AC, 16.7{{nbsp}}Hz]]
| 11{{nbsp}}kV
| 12{{nbsp}}kV
| 15{{nbsp}}kV
| 17.25{{nbsp}}kV
| 18{{nbsp}}kV
|-
| [[25 kV AC railway electrification|25{{nbsp}}kV AC, 50{{nbsp}}Hz (EN 50163) <br />and 60{{nbsp}}Hz (IEC 60850)]]
| 17.5{{nbsp}}kV
| 19{{nbsp}}kV
| 25{{nbsp}}kV
| 27.5{{nbsp}}kV
| 29{{nbsp}}kV
|}

== Direct current ==

=== Overhead lines ===
[[File:LGV Cruzilles Mépillat 10.jpg|thumb|The [[LGV Sud-Est]] in France is electrified using [[25 kV AC|{{nowrap|25 kV 50 Hz}}]] [[overhead lines]].]]
[[File:4,03 Hauts de St-Aubin Citadis n°1016 (tram Angers) par Cramos.JPG|thumb|right|The [[Angers tramway]] in Angers, France uses {{nowrap|750{{nbsp}}V{{nbsp}}DC}} overhead lines, in common with many other modern tram systems.]]
{{Main|Overhead line}}
1,500{{nbsp}}V DC is used in Japan, Indonesia, Hong Kong (parts), Ireland, Australia (parts), France (also using {{nowrap|{{25 kV 50 Hz}})}}, the Netherlands, New Zealand ([[Wellington]]), Singapore (on the [[North East MRT line]]), the United States ([[Chicago]] area on the [[Metra Electric Line|Metra Electric]] district and the [[South Shore Line (NICTD)|South Shore Line]] [[interurban]] line and [[Link light rail]] in [[Seattle]], Washington). In Slovakia, there are two narrow-gauge lines in the High Tatras (one a [[cog railway]]). In the Netherlands it is used on the main system, alongside 25{{nbsp}}kV on the [[HSL-Zuid]] and [[Betuwelijn]], and 3,000{{nbsp}}V south of [[Maastricht]]. In Portugal, it is used in the [[Cascais Line]] and in Denmark on the suburban [[S-train]] system (1650{{nbsp}}V DC).

In the United Kingdom, 1,500{{nbsp}}V{{nbsp}}DC was used in 1954 for the [[Manchester-Sheffield-Wath electric railway|Woodhead]] trans-Pennine route (now closed); the system used [[regenerative braking]], allowing for transfer of energy between climbing and descending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in [[East (London sub region)|East London]] and [[Manchester]], now converted to 25{{nbsp}}kV{{nbsp}}AC. It is now only used for the [[Tyne and Wear Metro]]. In India, 1,500{{nbsp}}V DC was the first electrification system launched in 1925 in Mumbai area. Between 2012 and 2016, the electrification was converted to 25{{nbsp}}kV 50{{nbsp}}Hz, which is the countrywide system.

3{{nbsp}}kV DC is used in Belgium, Italy, Spain, Poland, Slovakia, Slovenia, South Africa, Chile, the northern portion of the Czech Republic, the former republics of the [[Soviet Union]], and in the Netherlands on a few kilometers between Maastricht and Belgium. It was formerly used by the [[Milwaukee Road]] from [[Harlowton, Montana]], to Seattle, across the Continental Divide and including extensive branch and loop lines in Montana, and by the [[Delaware, Lackawanna and Western Railroad]] (now [[New Jersey Transit]], converted to 25{{nbsp}}kV{{nbsp}}AC) in the United States, and the [[Kolkata suburban railway]] (Bardhaman Main Line) in India, before it was converted to 25{{nbsp}}kV 50{{nbsp}}Hz.

DC voltages between 600{{nbsp}}V and 750{{nbsp}}V are used by most [[tram]]ways and [[trolleybus]] networks, as well as some [[Rapid transit|metro]] systems as the traction motors accept this voltage without the weight of an on-board transformer.{{Cn|date=April 2024}}

==== Medium-voltage DC ====
Increasing availability of high-voltage semiconductors may allow the use of higher and more efficient DC voltages that heretofore have only been practical with AC.<ref>P. Leandes and S. Ostlund. "A concept for an HVDC traction system" in "International conference on main line railway electrification", Hessington, England, September 1989 (Suggests 30{{nbsp}}kV). Glomez-Exposito A., Mauricio J.M., Maza-Ortega J.M. "VSC-based MVDC Railway Electrification System" IEEE transactions on power delivery, v. 29, no. 1, Feb. 2014. (suggests 24{{nbsp}}kV).</ref>

The use of medium-voltage DC electrification (MVDC) would solve some of the issues associated with standard-frequency AC electrification systems, especially possible supply grid load imbalance and the phase separation between the electrified sections powered from different phases, whereas high voltage would make the transmission more efficient.<ref name="Simiu&Davidson2021">{{cite journal |first1=Patrobers |last1=Simiyu |first2=I.E. |last2=Davidson |title=MVDC Railway Traction Power Systems; State-of-the Art, Opportunities, and Challenges |journal=Energies |year=2021 |volume=14 |issue=14 |page=4156 |publisher=MDPI |doi=10.3390/en14144156 |issn=1996-1073 |doi-access=free }}</ref>{{rp|6–7}} [[International Union of Railways|UIC]] conducted a case study for the conversion of the [[Bordeaux–Irun railway|Bordeaux-Hendaye railway line]] (France), currently electrified at 1.5{{nbsp}}kV DC, to 9{{nbsp}}kV DC and found that the conversion would allow to use less bulky overhead wires (saving €20 million per 100{{nbsp}}route-km) and lower the losses (saving 2{{nbsp}}GWh per year per 100{{nbsp}}route-km; equalling about €150,000 p.a.). The line chosen is one of the lines, totalling 6000{{nbsp}}km, that are in need of renewal.<ref>{{cite conference | url = https://uic.org/events/IMG/pdf/05-11_02_2019_uic_rotterdam.pdf | title = Future DC railway electrification system Go for 9kV | conference = UIC Workshop on Energy Efficiency | date = 11 September 2019 | location = Rotterdam | publisher = UIC }}</ref>

In the 1960s the Soviets experimented with boosting the overhead voltage from 3 to 6{{nbsp}}kV. DC rolling stock was equipped with [[ignitron]]-based converters to lower the supply voltage to 3{{nbsp}}kV. The converters turned out to be unreliable and the experiment was curtailed. In 1970 the [[Ural State University of Railway Transport|Ural Electromechanical Institute of Railway Engineers]] carried out calculations for railway electrification
at {{nowrap|12 kV DC}}, showing that the equivalent loss levels for a {{nowrap|25 kV AC}} system could be achieved with DC voltage between 11 and 16{{nbsp}}kV. In the 1980s and 1990s {{nowrap|12 kV DC}} was being tested on the [[October Railway]] near Leningrad (now [[Saint Petersburg|Petersburg]]). The experiments ended in 1995 due to the end of funding.<ref>{{Citation |first1=Б.А. |last1=Аржанников |first2=А.Г. |last2=Галкин |first3=А.Т. |last3=Бурков |first4=В.А. |last4=Мансуров |first5=И.О. |last5=Набойченко |title=Перспектива разработки системы электрической тяги постоянного тока повышенного напряжения 12, 24 кВ для скоростной магистрали Москва – Екатеринбург |journal=Бюллетень Результатов Научных Исследований |year=2015 |volume=1 |issue=14 |pages=38–44 |trans-title=Perspective of developing 12 or 24 kV DC electrification system for Moscow-Ekaterinburg high-speed line |url=https://cyberleninka.ru/article/n/perspektiva-razrabotki-sistemy-elektricheskoy-tyagi-postoyannogo-toka-povyshennogo-napryazheniya-12-24-kv-dlya-skorostnoy-magistrali |language=ru }}</ref>

=== Third rail ===
{{Main|Third rail}}
[[File:A metro station in Bucharest, showing it uses a 'third rail'.jpg|thumb|A bottom-contact third rail electrification system on the [[Bucharest Metro]], Romania]]
Most electrification systems use overhead wires, but third rail is an option up to 1,500{{nbsp}}V. Third rail systems almost exclusively use DC distribution. The use of AC is usually not feasible due to the dimensions of a third rail being physically very large compared with the [[skin effect|skin depth]] that AC penetrates to {{convert|0.3|mm|in|3|disp=or}} in a steel rail. This effect makes the resistance per unit length unacceptably high compared with the use of DC.<ref>Donald G. Fink, H. Wayne Beatty ''Standard Handbook for Electrical Engineers 11th Edition'', McGraw Hill, 1978 table 18-21. See also Gomez-Exposito p. 424, Fig. 3</ref> Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.{{Cn|date=April 2024}}

=== Fourth rail ===
{{See also|List of railway electrification systems#DC, four-rail}}
{{Unreferenced section|date=November 2022}}
[[File:EalingCommon3.jpg|thumb|The [[London Underground]] uses third and fourth rails beside and between the running rails for electrification.]]
The [[London Underground]] in England is one of few networks that uses a four-rail system.{{cn|date=December 2024}} The additional rail carries the electrical return that, on third-rail and overhead networks, is provided by the running rails. On the London Underground, a top-contact third rail is beside the track, energized at {{val|+420|u=V DC}}, and a top-contact fourth rail is located centrally between the running rails at {{val|-210|u=V DC}}, which combine to provide a traction voltage of {{nowrap|{{val|630|u=V}} DC}}. The same system was used for [[Milan]]'s earliest underground line, [[Milan Metro]]'s [[Milan Metro Line 1|line 1]], whose more recent lines use an overhead catenary or a third rail.

The key advantage of the four-rail system is that neither running rail carries any current. This scheme was introduced because of the problems of return currents, intended to be carried by the [[Ground (electricity)|earthed (grounded)]] running rail, flowing through the iron tunnel linings instead. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically [[Ground (electricity)|bonded]] together. The problem was exacerbated because the return current also had a tendency to flow through nearby iron pipes forming the water and gas mains. Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. The four-rail system solves the problem. Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels. Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails, which have to be seated on stronger metal chairs to carry the weight of trains. However, elastomeric rubber pads placed between the rails and chairs can now solve part of the problem by insulating the running rails from the current return should there be a leakage through the running rails.

The [[Expo Line (SkyTrain)|Expo]] and [[Millennium Line]] of the [[SkyTrain (Vancouver)|Vancouver SkyTrain]] use side-contact fourth-rail systems for their {{val|650|u=V DC}} supply. Both are located to the side of the train, as the space between the running rails is occupied by an aluminum plate, as part of stator of the [[Linear induction motor|linear induction propulsion]] system used on the [[Innovia Metro|Innovia ART]] system. While part of the SkyTrain network, the [[Canada Line]] does not use this system and instead uses more traditional motors attached to the wheels and third-rail electrification.

==== Rubber-tyred systems ====
{{Main|Rubber-tyred metro}}
[[Image:Bogie-metro-Meteor-p1010692.jpg|thumb|[[Bogie]] from an [[MP 89]] [[Paris Métro]] vehicle. The lateral [[contact shoe]] is located between the rubber tyres.]]
A few lines of the [[Paris Métro]] in France operate on a four-rail power system. The trains [[rubber-tyred metro|move on rubber tyres]] which roll on a pair of narrow [[roll way]]s made of steel and, in some places, of [[concrete]]. Since the tyres do not conduct the return current, the two [[guide bar]]s provided outside the running '[[roll way]]s' become, in a sense, a third and fourth rail which each provide {{nowrap|750 V DC}}, so at least electrically it is a four-rail system. Each [[wheelset (rail transport)|wheel set]] of a powered [[bogie]] carries one [[traction motor]]. A side sliding (side running) [[contact shoe]] picks up the current from the vertical face of each guide bar. The return of each traction motor, as well as each [[Railroad car|wagon]], is effected by one contact shoe each that slide on top of each one of the [[Rail profile|running rails]]. This and all other [[rubber-tyred metro]]s that have a {{Track gauge|sg|allk=on}} [[Track (rail transport)|track]] between the [[roll way]]s operate in the same manner.<ref>{{cite web|url=http://www.metro-pole.net/actu/article398.html|title=[MétroPole] De la centrale électrique au rail de traction|date=10 August 2004|url-status=dead|archive-url=https://web.archive.org/web/20040810033607/http://www.metro-pole.net/actu/article398.html|archive-date=10 August 2004}}</ref><ref>{{cite web|url=http://www.infovisual.info/05/050_en.html|title=Truck (bogie) – Visual Dictionary|first=Bernard|last=Dery|website=www.infovisual.info}}</ref>

== Alternating current ==
Railways and electrical utilities use AC as opposed to DC for the same reason:<ref>{{Cite web |title=MIT School of Engineering {{!}} » What's the difference between AC and DC? |url=https://engineering.mit.edu/engage/ask-an-engineer/whats-the-difference-between-ac-and-dc/ |access-date=2022-11-09 |website=Mit Engineering |language=en-US}}</ref> to use [[transformer]]s, which require AC, to produce higher voltages.<ref>{{Cite web |title=Alternating current – Energy Education |url=https://energyeducation.ca/encyclopedia/Alternating_current |access-date=2022-11-09 |website=energyeducation.ca}}</ref>
The higher the voltage, the lower the current for the same power (because power is current multiplied by voltage), and power loss is proportional to the current squared. The lower current reduces line loss, thus allowing higher power to be delivered.<ref>{{Cite web |title=What is Alternating Current (AC)? {{!}} Basic AC Theory {{!}} Electronics Textbook |url=https://www.allaboutcircuits.com/textbook/alternating-current/chpt-1/what-is-alternating-current-ac/ |access-date=2022-11-09 |website=www.allaboutcircuits.com |language=en}}</ref>

As alternating current is used with high voltages, inside the locomotive, a transformer steps the voltage down for use by the traction motors and auxiliary loads.

An early advantage of AC is that the power-wasting [[resistors]] used in DC locomotives for speed control were not needed in an AC locomotive: multiple taps on the [[transformer]] can supply a range of voltages.<ref>{{Cite web |title=[IRFCA] Electric Loco Tap-changer Operation |url=https://www.irfca.org/docs/tapchanger.html |access-date=2022-11-09 |website=www.irfca.org}}</ref>
Separate low-voltage transformer windings supply lighting and the motors driving auxiliary machinery.
More recently, the development of very high power semiconductors has caused the classic DC motor to be largely replaced with the three-phase [[induction motor]] fed by a [[variable frequency drive]], a special [[inverter]] that varies both frequency and voltage to control motor speed.
These drives can run equally well on DC or AC of any frequency, and many modern electric locomotives are designed to handle different supply voltages and frequencies to simplify cross-border operation.

=== Low-frequency alternating current ===
{{Main|15 kV AC railway electrification|Amtrak's 25 Hz traction power system|SEPTA's 25 Hz traction power system}}
[[File:ÖBB Vectron 1293 015 - Bhf Rattenberg - Tirol - AT.jpg|thumb|An [[ÖBB]] train using 15{{nbsp}}kV 16.7{{nbsp}}Hz AC overhead lines in Austria]]
Five European countries {{ndash}} Germany, Austria, Switzerland, Norway and Sweden {{ndash}} have standardized on 15{{nbsp}}kV {{frac|16|2|3}}{{nbsp}}Hz (the 50{{nbsp}}Hz mains frequency divided by three) single-phase AC. On 16 October 1995, Germany, Austria and Switzerland changed from {{frac|16|2|3}}{{nbsp}}Hz to 16.7{{nbsp}}Hz which is no longer exactly one-third of the grid frequency. This solved overheating problems with the [[rotary converter]]s used to generate some of this power from the grid supply.<ref>{{cite book | first=C. | last=Linder | title=Umstellung der Sollfrequenz im zentralen Bahnstromnetz von 16 2/3 Hz auf 16,70 Hz |trans-title=Switching the frequency in train electric power supply network from 16 2/3 Hz to 16,70 Hz | series=Elektrische Bahnen | issue=Book 12 | publisher=Oldenbourg-Industrieverlag |year=2002 | issn=0013-5437 | language=de}}</ref>

[[Railroad electrification in the United States|In the US]], the [[Electrification of the New York, New Haven, and Hartford Railroad|New York, New Haven, and Hartford Railroad]], the [[Pennsylvania Railroad#Electrification|Pennsylvania Railroad]] and the [[Reading Company#Philadelphia and Reading Railway: 1896–1923|Philadelphia and Reading Railway]] adopted 11{{nbsp}}kV 25{{nbsp}}Hz single-phase AC. Parts of the original electrified network still operate at 25{{nbsp}}Hz, with voltage boosted to 12{{nbsp}}kV, while others were converted to 12.5 or 25{{nbsp}}kV 60{{nbsp}}Hz.<!--see wikilinked articles for sources-->

In the UK, the [[London, Brighton and South Coast Railway]] pioneered overhead electrification of its suburban lines in London, [[London Bridge station|London Bridge]] to [[London Victoria station|Victoria]] being opened to traffic on 1{{nbsp}}December 1909. Victoria to [[Crystal Palace railway station|Crystal Palace]] via Balham and West Norwood opened in May 1911. [[Peckham Rye railway station|Peckham Rye]] to [[West Norwood railway station|West Norwood]] opened in June 1912. Further extensions were not made owing to the First World War. Two lines opened in 1925 under the [[Southern Railway (Great Britain)|Southern Railway]] serving [[Coulsdon North railway station|Coulsdon North]] and [[Sutton railway station (London)|Sutton railway station]]. The lines were electrified at 6.7{{nbsp}}kV 25{{nbsp}}Hz. It was announced in 1926 that all lines were to be converted to DC third rail and the last overhead-powered electric service ran in September 1929.

=== Standard frequency alternating current ===
[[File:Onderdel spoorwegnet.gif|thumb|350px|Schema of 2×25&nbsp;kV power supply:<br>1. Supply transformer (center-tapped output)<br>2. Power supply<br>3. Overhead line<br>4. Running rail<br>5. Feeder line<br>6. Pantograph<br>7. Locomotive transformer<br>8. Overhead line<br>9. Autotransformer<br>10. Running rail]]
{{Main|25 kV AC railway electrification}}
AC power is used at 60{{nbsp}}Hz in North America (excluding the aforementioned 25{{nbsp}}Hz network), Saudi Arabia,<ref>{{cite news |last1=Torga |first1=Ferran |title=Haramain high-speed crawls towards the start of operations |url=https://www.railjournal.com/in_depth/haramain-high-speed-crawls-towards-the-start-of-operations/ |access-date=1 August 2024 |work=International Railway Journal |date=February 14, 2017}}</ref><ref name="ICCP">{{cite web |last1=Alvarez-Maldonado Paramés |first1=Javier |title=Haramain High Speed Railway: Fase II. Alcance del Proyecto |url=http://www.caminosmadrid.es/es/Demarcacion/II_SemanaIngenieria/ponenciaspdf/JavierAlvarez12032013.pdf |publisher={{interlanguage link|Colegio de Ingenieros de Caminos, Canales y Puertos|es}} |archive-url=https://web.archive.org/web/20141227203951/http://www.caminosmadrid.es/es/Demarcacion/II_SemanaIngenieria/ponenciaspdf/JavierAlvarez12032013.pdf |archive-date=2014-12-27 |date=12 March 2013 |language=es |url-status=dead}}</ref> western Japan, South Korea, and Taiwan; and at 50{{nbsp}}Hz in a number of European countries, India, eastern Japan, countries that used to be part of the Soviet Union, on high-speed lines in much of Western Europe (including countries that still run conventional railways under DC but not in countries using 16.7{{nbsp}}Hz, see above). Most systems like this operate at 25{{nbsp}}kV, although 12.5{{nbsp}}kV sections exist in the United States, and 20{{nbsp}}kV is used on some lines in Japan. On "French system" HSLs, the overhead line and a "sleeper" feeder line each carry 25{{nbsp}}kV in relation to the rails, but in opposite phase so they are at 50{{nbsp}}kV from each other; [[autotransformer]]s equalize the tension at regular intervals.{{Citation needed|date=November 2022}}

=== Three-phase alternating current ===
{{Main|Three-phase AC railway electrification}}
[[File:2012-11-03 BDhe48 218-211 04 Eigergletscher.jpg|thumb|left|The [[Jungfrau Railway]], the highest in Europe, uses three-phase electric power between two overhead lines and the rails.]]
Various railway electrification systems in the late nineteenth and twentieth centuries utilised [[Three-phase electric power|three-phase]], rather than single-phase electric power delivery due to ease of design of both power supply and locomotives. These systems could either use standard network frequency and three power cables, or reduced frequency, which allowed for return-phase line to be third rail, rather than an additional overhead wire.{{Citation needed|date=November 2022}}

== Comparisons ==

=== AC versus DC for mainlines ===
The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive, and within the locomotive, [[Transformer|transformed]] and [[Rectifier|rectified]] to a lower DC voltage in preparation for use by traction motors. These motors may either be DC motors which directly use the DC or they may be three-phase AC motors which require further conversion of the DC to variable frequency three-phase AC (using power electronics). Thus both systems are faced with the same task: converting and transporting high-voltage AC from the power grid to low-voltage DC in the locomotive. The difference between AC and DC electrification systems lies in where the AC is converted to DC: at the substation or on the train. Energy efficiency and infrastructure costs determine which of these is used on a network, although this is often fixed due to pre-existing electrification systems.

Both the transmission and conversion of electric energy involve losses: ohmic losses in wires and power electronics, magnetic field losses in transformers and smoothing reactors (inductors).<ref>See Винокуров p. 95+ Ch. 4: Потери и коэффициент полизного действия; нагреванние и охлаждение электрических машин и трансформаторов" (Losses and efficiency; heating and cooling of electrical machinery and transformers) magnetic losses pp. 96–97, ohmic losses pp. 97–99</ref> Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher.<ref>Сидоров 1988 pp. 103–104, Сидоров 1980 pp. 122–123</ref> However, the higher voltages used in many AC electrification systems reduce transmission losses over longer distances, allowing for fewer substations or more powerful locomotives to be used. Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for.

Standard AC electrification systems use much higher voltages than standard DC systems. One of the advantages of raising the voltage is that, to transmit certain level of power, lower current is necessary ({{math|''P'' {{=}} ''V'' × ''I''}}). Lowering the current reduces the ohmic losses and allows for less bulky, lighter overhead line equipment and more spacing between traction substations, while maintaining power capacity of the system. On the other hand, the higher voltage requires larger isolation gaps, requiring some elements of infrastructure to be larger. The standard-frequency AC system may introduce imbalance to the supply grid, requiring careful planning and design (as at each substation power is drawn from two out of three phases). The low-frequency AC system may be powered by [[Traction power network|separate generation and distribution network]] or a network of converter substations, adding the expense, also low-frequency transformers, used both at the substations and on the rolling stock, are particularly bulky and heavy. The DC system, apart from being limited as to the maximum power that can be transmitted, also can be responsible for electrochemical corrosion due to stray DC currents.{{r|Simiu&Davidson2021|p=3}}

=== Electric versus diesel ===
[[File:Thomas Robert Way00.jpg|thumb|upright|[[Lots Road Power Station]] in a poster from 1910. This private power station, used by [[London Underground]], gave London trains and trams a power supply independent from the main power network.]]

==== Energy efficiency ====
Electric trains need not carry the weight of [[prime mover (locomotive)|prime movers]], transmission and fuel. This is partly offset by the weight of electrical equipment.
[[Regenerative braking]] returns power to the electrification system so that it may be used elsewhere, by other trains on the same system or returned to the general power grid. This is especially useful in mountainous areas where heavily loaded trains must descend long grades.<ref>{{Cite web |title=Unlocking the full benefits of rail electrification – Future Rail {{!}} Issue 98 {{!}} September 2022 |url=https://rail.nridigital.com/future_rail_sep22/ail_electrification_benefits |access-date=2023-02-17 |website=rail.nridigital.com |language=en}}</ref>

Central station electricity can often be generated with higher efficiency than a mobile engine/generator. While the efficiency of power plant generation and diesel locomotive generation are roughly the same in the nominal regime,<ref>It turns out that the efficiency of electricity generation by a modern diesel locomotive is roughly the same as the typical U.S. fossil-fuel power plant. The heat rate of central power plants in 2012 was about 9.5k BTU/kwh per the Monthly Energy Review of the U.S. Energy Information Administration which corresponds to an efficiency of 36%. Diesel motors for locomotives have an efficiency of about 40% (see [[Brake specific fuel consumption]], Дробинский p. 65 and Иванова p.20.). But there are reductions needed in both efficiencies needed to make a comparison. First, one must degrade the efficiency of central power plants by the transmission losses to get the electricity to the locomotive. Another correction is due to the fact that efficiency for the Russian diesel is based on the lower heat of combustion of fuel while power plants in the U.S. use the higher heat of combustion (see [[Heat of combustion]]). Still another correction is that the diesel's reported efficiency neglects the fan energy used for engine cooling radiators. See Дробинский p. 65 and Иванова p. 20 (who estimates the on-board electricity generator as 96.5% efficient). The result of all the above is that modern diesel engines and central power plants are both about 33% efficient at generating electricity (in the nominal regime).</ref> diesel motors decrease in efficiency in non-nominal regimes at low power<ref>Хомич А. З. Тупицын О.И., Симсон А. Э. "Экономия топлива и теплотехническая модернизация тепловозов" (Fuel economy and the thermodynamic modernization of diesel locomotives). Москва: Транспорт, 1975. 264 pp. See [[Brake specific fuel consumption]] curves on p. 202 and charts of times spent in non-nominal regimes on pp. 10–12</ref> while if an electric power plant needs to generate less power it will shut down its least efficient generators, thereby increasing efficiency. The electric train can save energy (as compared to diesel) by [[regenerative braking]] and by not needing to consume energy by idling as diesel locomotives do when stopped or coasting. However, electric rolling stock may run cooling blowers when stopped or coasting, thus consuming energy.

Large [[fossil fuel power station]]s operate at high efficiency, and can be used for [[district heating]] or to produce [[district cooling]], leading to a higher total efficiency.<ref>{{cite web|last=Wang |first=Ucilia |url=http://gigaom.com/cleantech/ge-to-crank-up-gas-power-plants-like-jet-engines/ |title=Gigaom GE to Crank Up Gas Power Plants Like Jet Engines |publisher=Gigaom.com |date=2011-05-25 |access-date=2016-02-04}}</ref><ref>{{usurped|1=[https://web.archive.org/web/20120824224826/http://www.ge-flexibility.com/solutions/flexefficiency-50-combined-cycle-power-plant/index.html FlexEfficiency* 50 Combined Cycle Power Plant]}}</ref> Electricity for electric rail systems can also come from [[renewable energy]], [[nuclear power]], or other low-carbon sources, which do not emit pollution or emissions.

==== Power output ====
Electric locomotives may easily be constructed with greater power output than most diesel locomotives. For passenger operation it is possible to provide enough power with diesel engines (see e.g. '[[ICE TD]]') but, at higher speeds, this proves costly and impractical. Therefore, almost all [[high speed train]]s are electric. The high power of electric locomotives also gives them the ability to pull freight at higher speed over gradients; in mixed traffic conditions this increases capacity when the time between trains can be decreased. The higher power of electric locomotives and an electrification can also be a cheaper alternative to a new and less steep railway if train weights are to be increased on a system.

On the other hand, electrification may not be suitable for lines with low frequency of traffic, because lower running cost of trains may be outweighed by the high cost of the electrification infrastructure. Therefore, most long-distance lines in developing or sparsely populated countries are not electrified due to relatively low frequency of trains.

==== Network effect ====
[[Network effect]]s are a large factor with electrification.{{citation needed|date=July 2017}} When converting lines to electric, the connections with other lines must be considered. Some electrifications have subsequently been removed because of the through traffic to non-electrified lines.{{citation needed|date=July 2017}} If through traffic is to have any benefit, time-consuming engine switches must occur to make such connections or expensive [[electro-diesel locomotive|dual mode engines]] must be used. This is mostly an issue for long-distance trips, but many lines come to be dominated by through traffic from long-haul freight trains (usually running coal, ore, or containers to or from ports). In theory, these trains could enjoy dramatic savings through electrification, but it can be too costly to extend electrification to isolated areas, and unless an entire network is electrified, companies often find that they need to continue use of diesel trains even if sections are electrified. The increasing demand for container traffic, which is more efficient when utilizing the [[double-stack car]], also has network effect issues with existing electrifications due to insufficient clearance of overhead electrical lines for these trains, but electrification can be built or modified to have sufficient clearance, at additional cost.

A problem specifically related to electrified lines are gaps in the electrification. Electric vehicles, especially locomotives, lose power when traversing gaps in the supply, such as phase change gaps in overhead systems, and gaps over points in third rail systems. These become a nuisance if the locomotive stops with its collector on a dead gap, in which case there is no power to restart. This is less of a problem in trains consisting of two or more [[multiple unit]]s [[Railway coupling|coupled]] together, since in that case if the train stops with one collector in a dead gap, another multiple unit can push or pull the disconnected unit until it can again draw power. The same applies to the kind of [[push-pull train]]s which have a locomotive at each end. Power gaps can be overcome in single-collector trains by on-board batteries or motor-flywheel-generator systems.{{citation needed|date=July 2017}}
In 2014, progress is being made in the use of large [[capacitor]]s to power electric vehicles between stations, and so avoid the need for overhead wires between those stations.<ref>[[Railway Gazette International]] Oct 2014.</ref>

==== Maintenance costs ====
Maintenance costs of the lines may be increased by electrification, but many systems claim lower costs due to reduced wear-and-tear on the track from lighter rolling stock.<ref>[http://www.networkrail.co.uk/browse%20documents/rus%20documents/route%20utilisation%20strategies/network/working%20group%204%20-%20electrification%20strategy/networkrus_electrification.pdf "UK Network Rail electrification strategy report"] {{Webarchive|url=https://web.archive.org/web/20130622210143/http://www.networkrail.co.uk/browse%20documents/rus%20documents/route%20utilisation%20strategies/network/working%20group%204%20-%20electrification%20strategy/networkrus_electrification.pdf |date=22 June 2013 }} Table 3.3, p. 31. Retrieved on 4 May 2010</ref> There are some additional maintenance costs associated with the electrical equipment around the track, such as power sub-stations and the catenary wire itself, but, if there is sufficient traffic, the reduced track and especially the lower engine maintenance and running costs exceed the costs of this maintenance significantly.

==== Sparks effect ====
Newly electrified lines often show a "sparks effect", whereby electrification in passenger rail systems leads to significant jumps in patronage / revenue.<ref name=STARTSLOW>{{Cite news|title=Start Slow With Bullet Trains|url=http://www.miller-mccune.com/politics/start-slow-with-bullet-trains-29853/|access-date=27 February 2012|newspaper=Miller-McCune|date=2 May 2011|url-status=dead|archive-url=https://web.archive.org/web/20120128054303/http://www.miller-mccune.com/politics/start-slow-with-bullet-trains-29853/|archive-date=28 January 2012}}</ref> The reasons may include electric trains being seen as more modern and attractive to ride,<ref name=ONTRACKFOR>{{Cite news|title=Cumbernauld may be on track for railway line electrification|url=http://www.cumbernauld-news.co.uk/news/local-headlines/cumbernauld_may_be_on_track_for_railway_line_electrification_1_353988|archive-url=https://archive.today/20130419232749/http://www.cumbernauld-news.co.uk/news/local-headlines/cumbernauld_may_be_on_track_for_railway_line_electrification_1_353988|url-status=dead|archive-date=19 April 2013|access-date=27 February 2012|newspaper=Cumbernauld News|date=14 January 2009}}</ref><ref>{{Cite news|title=Electric Idea|url=http://www.bromsgroveadvertiser.co.uk/news/1949797.electric_idea/|access-date=27 February 2012|newspaper=Bromsgrove Advertiser|date=8 January 2008}}</ref> faster, quieter and smoother service,<ref name=STARTSLOW/> and the fact that electrification often goes hand in hand with a general infrastructure and rolling stock overhaul / replacement, which leads to better service quality (in a way that theoretically could also be achieved by doing similar upgrades yet without electrification). Whatever the causes of the sparks effect, it is well established for numerous routes that have electrified over decades.<ref name=STARTSLOW/><ref name=ONTRACKFOR/> This also applies when bus routes with diesel buses are replaced by trolleybuses. The overhead wires make the service "visible" even if no bus is running and the existence of the infrastructure gives some long-term expectations of the line being in operation.

==== Double-stack rail transport ====
{{See also|Double-stack rail transport}}
Due to the [[Loading gauge|height restriction]] imposed by the overhead wires, double-stacked container trains have been traditionally difficult and rare to operate under electrified lines. However, this limitation is being overcome by railways in India, China and African countries by laying new tracks with increased catenary height.

Such installations are in the [[Western Dedicated Freight Corridor]] in India where the wire height is at {{convert|7.45|m|abbr=on}} to accommodate double-stack container trains without the need of [[Class U special wagon|well-wagons]].

==== Advantages ====
{{More citations needed|section|date=February 2023}}
There are a number of advantages including the fact there is no exposure of passengers to exhaust from the locomotive and lower cost of building, running and maintaining locomotives and [[multiple unit]]s. Electric trains have a higher power-to-weight ratio (no onboard fuel tanks), resulting in fewer locomotives, faster acceleration, higher practical limit of power, higher limit of speed, less [[noise pollution]] (quieter operation). The faster acceleration clears lines more quickly to run more trains on the track in urban rail uses.<ref>{{Cite web |title=Rail – Analysis |url=https://www.iea.org/reports/rail |access-date=2023-02-17 |website=IEA |language=en-GB}}</ref>
* Reduced power loss at higher altitudes (for ''power loss'' see [[Diesel engine]])
* Independence of running costs from fluctuating fuel prices
* Service to underground stations where diesel trains cannot operate for safety reasons
* Reduced environmental pollution, especially in highly populated urban areas, even if electricity is produced by [[fossil fuels]]
* Easily accommodates kinetic energy brake reclaim using supercapacitors
* More comfortable ride on multiple units as trains have no underfloor diesel engines
* Somewhat higher energy efficiency <ref>Per [[Railway electrification in the Soviet Union#Energy-Efficiency]] it was claimed that after the mid 1970s electrics used about 25% less fuel per ton-km than diesels. However, part of this savings may be due to less stopping of electrics to let opposing trains pass since diesels operated predominately on single-track lines, often with moderately heavy traffic.</ref> in part due to [[regenerative braking]] and less power lost when "idling"
* More flexible primary energy source: can use coal, natural gas, nuclear or renewable energy (hydro, solar, wind) as the primary energy source instead of diesel fuel
* If the entire network is electrified, diesel infrastructure such as fueling stations, maintenance yards and indeed the diesel locomotive fleet can be retired or put to other uses – this is often the business case in favor of electrifying the last few lines in a network where otherwise costs would be too high. Having only one type of motive power also allows greater fleet homogeneity which can also reduce costs.

==== Disadvantages ====
[[File:Berwick-upon-Tweed MMB 14 Royal Border Bridge.jpg|thumb|The [[Royal Border Bridge]] in [[England]], a [[Listed building|protected monument]]. Adding electric catenary to older structures may be an expensive cost of electrification projects.]]
[[File:DTTX 724681 20050529 IL Rochelle.jpg|thumb|Many electrification systems using overhead lines do not allow sufficient [[Engineering tolerance#Clearance (civil engineering)|clearance]] for a [[Well car|double-stack car]]. Each [[Intermodal container|container]] may be {{convert|9|ft|6+1/2|in|abbr=on}} tall and the bottom of the well is {{convert|1|ft|2|in|abbr=on}} above [[Rail profile|rail]], making the [[Loading gauge#Freight|overall height]] {{convert|20|ft|3|in|abbr=on}} including the well car.<ref>[http://www.emdx.org/rail/Gabarit/index.html] AAR Plate H</ref>]]
* Electrification cost: electrification requires an entire new infrastructure to be built around the existing tracks at a significant cost. Costs are especially high when tunnels, bridges and other [[Structure gauge|obstructions]] have to be altered for [[Engineering tolerance#Clearance (civil engineering)|clearance]]. Another aspect that can raise the cost of electrification are the alterations or upgrades to [[railway signalling]] needed for new traffic characteristics, and to protect signalling circuitry and [[track circuits]] from interference by traction current. Electrification typically requires line closures while new equipment is being installed.
* Appearance: the overhead line structures and cabling can have a significant landscape impact compared with a non-electrified or third rail electrified line that has only occasional signalling equipment above ground level.
* Fragility and vulnerability: overhead electrification systems can suffer severe disruption due to minor mechanical faults or the effects of high winds causing the [[Pantograph (transport)|pantograph]] of a moving train to become entangled with the [[Overhead line#Overhead catenary|catenary]], ripping the wires from their supports. The damage is often not limited to the supply to one track, but extends to those for adjacent tracks as well, causing the entire route to be blocked for a considerable time. Third-rail systems can suffer disruption in cold weather due to ice forming on the conductor rail.<ref name="metoffice">{{cite web |url=http://www.rmets.org/sites/default/files/mos_news_spr2009.pdf |title=Committee Meeting – Spring 2009 |publisher=[[Royal Meteorological Society]] (rmets.org) |access-date=15 September 2012 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304032324/http://www.rmets.org/sites/default/files/mos_news_spr2009.pdf |url-status=dead }}</ref>
* Theft: the high scrap value of copper and the unguarded, remote installations make overhead cables an attractive target for scrap metal thieves.<ref name="nrcable">{{cite web |url=http://www.networkrail.co.uk/cabletheft/ |title=Network Rail – Cable Theft |publisher=[[Network Rail]] (www.networkrail.co.uk) |access-date=15 September 2012}}</ref> Attempts at theft of live 25{{nbsp}}kV cables may end in the thief's death from electrocution.<ref name="itvcable">{{cite web |url=http://www.itv.com/news/meridian/story/2012-06-27/police-probe-cable-theft-death-link/ |work=ITV News |title=Police probe cable theft death link |date=27 June 2012 |access-date=15 September 2012}}</ref> In the UK, cable theft is claimed to be one of the biggest sources of delay and disruption to train services – though this normally relates to signalling cable, which is equally problematic for diesel lines.<ref name="itvcable2">{{Cite news |author=Sarah Saunders |work=ITV News |url=http://www.itv.com/news/meridian/2012-06-27/body-discovery-linked-to-rail-cables-theft/ |title=Body discovery linked to rail cables theft |date=28 June 2012 |access-date=7 May 2014}}</ref>
* Incompatibility: Diesel trains can run on any track without electricity or with any kind of electricity ([[third rail]] or [[overhead line]], DC or AC, and at any voltage or frequency). Not so for electric trains, which can never run on non-electrified lines, and which even on electrified lines can run only on the single, or the few, electrical system(s) for which they are equipped. Even on fully electrified networks, it is usually a good idea to keep a few diesel locomotives for maintenance and repair trains, for instance to repair broken or stolen overhead lines, or to lay new tracks. However, due to ventilation issues, diesel trains may have to be banned from certain tunnels and underground train stations mitigating the advantage of diesel trains somewhat.
* Birds may perch on parts with different charges, and animals may also touch the electrification system. Dead animals attract foxes or other scavengers,<ref>{{cite web |url=https://www.nabu.de/tiere-und-pflanzen/voegel/gefaehrdungen/stromtod/ |title=Tiere & Pflanzen Vögel Gefährdungen Stromtod Mehr aus dieser Rubrik Vorlesen Die tödliche Gefahr |work=[[Naturschutzbund Deutschland|Naturschutzbund]] |location=Berlin, Germany |first1=Lars |last1=Nachmann|access-date=20 July 2016 |language=de}}</ref> bringing risk of collision with trains.
* In most of the world's railway networks, the height clearance of overhead electrical lines is not sufficient for a double-stack container car or other unusually tall loads. To upgrade electrified lines to the correct clearances ({{convert|21|ft|8|in|abbr=on|disp=or}}) to take double-stacked container trains, besides renewing bridges over it, would normally mean need for special [[Pantograph (transport)|pantographs]] violating standardisation and requiring custom made vehicles{{citation needed|date=January 2023}}.

== Railway electrification around the world ==
{{See also|List of countries by railway electrification}}
As of 2012, electrified tracks accounted for nearly one third of total tracks globally.<ref name="auto"/>

As of 2018, there were {{cvt|72110|km|mi|-1}} of railways electrified at 25{{nbsp}}kV, either 50 or 60{{nbsp}}Hz; {{cvt|68890|km|mi|-1}} electrified at {{nowrap|3 kV DC}}; {{cvt|32940|km|mi|-1}} electrified at 15{{nbsp}}kV 16.7 or {{frac|16|2|3}}{{nbsp}}Hz and {{cvt|20440|km|mi|-1}} electrified at {{nowrap|1.5 kV DC}}.{{r|Simiu&Davidson2021|p=2}}

As of 2023, the [[Rail transport in Switzerland|Swiss rail network]] is the largest fully electrified network in the world and one of only eleven countries or territories to achieve this, as listed in [[List of countries by rail transport network size]]. The percentage then continues falling in order with Laos, Montenegro, India, Belgium, Georgia, South Korea, Netherlands, and Japan, with all others being less than 75% electrified. 
<ref>{{Cite web|title=2019 年铁道统计公报|url=http://www.mot.gov.cn/tongjishuju/tielu/202005/P020200511352537906795.pdf|access-date=7 June 2020|archive-date=6 June 2020|archive-url=https://web.archive.org/web/20200606235351/http://www.mot.gov.cn/tongjishuju/tielu/202005/P020200511352537906795.pdf|url-status=dead}}</ref><ref>{{Cite web |title=Status of Railway Electrification (as on 01.04.2023) |url=https://indianrailways.gov.in/railwayboard/uploads/directorate/ele_engg/2023/Railway%20Electrification%20as%20on%2001_04_23.pdf}}</ref> Overall, China takes first place, with around {{cvt|100000|km|}} of electrified railway, followed by India with over {{cvt|60000|km}} of electrified railway, and continuing with Russia, with over {{cvt|54000|km}} of electrified railway. A number of countries have zero electrified railways, instead relying on diesel multiple units, locomotive hauled services and many alternate forms of transport. The European Union contains the longest amount of electrified railways (in length), with over {{cvt|114000|km}} of electrified railway, however only making up around 55% of the total railway length.

Several countries have announced plans to electrify all or most of their railway network, including [[Indian Railways]] and [[Israel Railways]].<ref>{{Cite web |title=On track to full electrification: Low carbon railways in India |url=https://www.rapidtransition.org/stories/on-track-to-full-electrification-low-carbon-railways-in-india/ |access-date=2023-02-17 |website=www.rapidtransition.org |language=en-GB}}</ref>

The [[Trans-Siberian Railway]] mainly in Russia is completely electrified, making it one of the longest stretches of electrified railways in the world.<ref>{{Cite web |date=2015-09-04 |title=Russia's legendary Trans-Siberian railroad line completely electrified – AP Worldstream {{!}} HighBeam Research |url=http://www.highbeam.com/doc/1P1-70603200.html |access-date=2022-11-09 |archive-url=https://web.archive.org/web/20150904061530/http://www.highbeam.com/doc/1P1-70603200.html |archive-date=4 September 2015 }}</ref>

== See also ==
{{portal|Energy|Trains}}
{{div col|colwidth=20em}}
* [[Battery electric multiple unit]]
* [[Battery locomotive]]
* [[Conduit current collection]]
* [[Current collector]]
* [[Dual electrification]]
* [[Electromote]]
* [[Line 3 Scarborough#Track|Fifth rail system]]
* [[Ground-level power supply]]
* [[Electric locomotive#History|History of the electric locomotive]]
* [[Electrification of the New York, New Haven, and Hartford Railroad#Initial Electrification Experiments|Initial Electrification Experiments NY NH HR]]
* [[List of railway electrification systems]]
* [[List of tram systems by gauge and electrification]]
* [[Multi-system (rail)]]
* [[Overhead line#Overhead conductor rails|Overhead conductor rails]]
* [[Railroad electrification in the United States]]
* [[Stud contact system]]
* [[Traction current pylon]]
* [[Traction powerstation]]
* [[Traction substation]]
{{div col end}}

== References ==
{{reflist}}

== Further reading ==
* {{Cite book |last=Wilner |first=Frank |title=Amtrak: Past, Present, Future |publisher=Simmons-Boardman Books, Omaha |date=2012 |isbn=978-0911382-59-4}}
* {{Cite journal|date=January 2021|title=On board with electrification|journal=Permanent Way Institution Journal|volume=139|issue=1|issn=2057-2425|via=PWI}}
* {{Cite book|last=Keenor|first=Garry|title=Overhead Line Electrification for Railways |edition=6th|url=https://www.ocs4rail.com |isbn=978-0-903489-15-7}}
* {{Cite web|date=February 2015|title=Network Rail A Guide to Overhead Electrification Revision 10|url=https://www.bathnes.gov.uk/sites/default/files/sitedocuments/Planning-and-Building-Control/Planning/nr_a_guide_to_overhead_electrification.pdf|website=Network Rail}}
* {{cite book | last=Nock | first=O.S.| title= Britain's new railway: Electrification of the London-Midland main lines from Euston to Birmingham, Stoke-on-Trent, Crewe, Liverpool and Manchester| publisher=Ian Allan | location = London| year=1965|oclc=59003738}}
* {{Cite book|last=Nock|first=O.S.|title=Electric Euston to Glasgow|publisher=Ian Allan|year=1974|isbn=978-0711005303}}
* {{cite book|last=Walker|first=Robert|title=The Trans-Siberian Railway Encyclopedia| url = https://trans-siberian-railway-encyclopedia.com/| archive-url = https://web.archive.org/web/20170712015856/https://trans-siberian-railway-encyclopedia.com/| url-status = usurped| archive-date = 12 July 2017}}
* {{cite book|last=Wolmar|first=Christian|title=To the Edge of the World: The Story of the Trans-Siberian Express, the World's Greatest Railroad| location = London|publisher=Atlantic Books|year=2013|isbn=978-0857890375}}
* {{Cite web |title=Electrification of Indian Railways to Lower Carbon Footprint |url=https://www.investindia.gov.in/team-india-blogs/electrification-indian-railways-lower-carbon-footprint |access-date=2022-11-09 |website=www.investindia.gov.in |language=en}}
* {{Cite web |title=Rail Electrification |url=https://www.mobility.siemens.com/global/en/portfolio/rail/electrification.html |access-date=2023-02-17 |website=Siemens Mobility Global |language=en}}

== Sources ==

=== English ===
* {{cite book | first = G T| last = Moody| year = 1960| title = Southern Electric| chapter = Part One| edition = 3rd| publisher =Ian Allan Ltd| location = London }}
* Gomez-Exposito A., Mauricio J.M., Maza-Ortega J.M. "VSC-based MVDC Railway Electrification System" IEEE transactions on power delivery, v. 29, no. 1, Feb. 2014 pp.{{nbsp}}422–431. (suggests {{nowrap|24 kV DC}})
* {{usurped|1=[https://web.archive.org/web/20030815044443/http://juts.janes.com/public/juts/index.shtml (Jane's) Urban Transit Systems]}}
* {{cite book|last=Hammond|first=John Winthrop|title=Men and volts; the story of General Electric|url=https://archive.org/details/menandvoltsstory00hammrich|year=2011|location=Philadelphia, Pennsylvania; London|publisher=General Electric Company; [[J. B. Lippincott & Co.]]; Literary Licensing, LLC|isbn=978-1-258-03284-5|orig-year=1941 |quote=He was to produce the first motor that operated without gears of any sort, having its armature direct-connected to the car axle. |via=[[Internet Archive]] }}
* {{cite book |editor-last1=Kaempffert |editor-first1=Waldemar Bernhard  |author-link=Waldemar Kaempffert|url=https://archive.org/details/popularhistoryof01kaem |first=T. Commerford |last=Martin |year=1924 |title=A Popular History of American Invention |via=[[Internet Archive]] |publisher=[[Charles Scribner's Sons]]|location=London; New York|volume=1 }}
* {{cite book|last=Malone|first=Dumas |author-link=Dumas Malone|title=Sidney Howe Short |url=https://books.google.com/books?id=fxFQAQAAMAAJ&pg=PA128|year=1928|publisher=[[Charles Scribner's Sons]] |location=London; New York|volume=17}}

=== Russian ===
* Винокуров В.А., Попов Д.А. "Электрические машины железно-дорожного транспорта" (Electrical machinery of railroad transportation), Москва, Транспорт, 1986. {{ISBN|5-88998-425-X}}, 520 pp.
* Дмитриев, В.А., "Народнохозяйственная эффективность электрификации железных дорог и применения тепловозной тяги" (National economic effectiveness of railway electrification and application of diesel traction), Москва, Транспорт 1976.
* Дробинский В.А., Егунов П.М. "Как устроен и работает тепловоз" (How the diesel locomotive works) 3rd ed. Moscow, Транспорт, 1980.
* Иванова В.Н. (ed.) "Конструкция и динамика тепловозов" (Construction and dynamics of the diesel locomotive). Москва, Транспорт, 1968 (textbook).
* Калинин, В.К. "Электровозы и электропоезда" (Electric locomotives and electric train sets) Москва, Транспорт, 1991 {{ISBN|978-5-277-01046-4}}
* Мирошниченко, Р.И., "Режимы работы электрифицированных участков" (Regimes of operation of electrified sections [of railways]), Москва, Транспорт, 1982.
* Перцовский, Л. М.; "Энергетическая эффективность электрической тяги" (Energy efficiency of electric traction), ''Железнодорожный транспорт'' (magazine), #12, 1974 p.{{nbsp}}39+
* Плакс, А.В. & Пупынин, В. Н., "Электрические железные дороги" (Electric Railways), Москва "Транспорт" 1993.
* Сидоров Н.И., Сидорожа Н.Н. "Как устроен и работает электровоз" (How the electric locomotive works) Москва, Транспорт, 1988 (5th ed.). 233 pp, {{ISBN|978-5-277-00191-2}}. 1980 (4th ed.).
* Хомич А.З. Тупицын О.И., Симсон А.Э. "Экономия топлива и теплотехническая модернизация тепловозов" (Fuel economy and the thermodynamic modernization of diesel locomotives). Москва: Транспорт, 1975. 264 pp.

== External links ==
* {{Commons category-inline|Electrically-powered rail transport}}

{{Railway electrification}}
{{Rail tracks}}

{{Authority control}}

{{DEFAULTSORT:Railway electrification}}
[[Category:Electric rail transport]]
[[Category:Rail transport]]
[[Category:Trains]]