Railway electrification system

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Electric locomotives under the wires in Sweden.

A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electrification has many advantages but requires significant capital expenditure. 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 locomotives can switch to different supply voltages to allow flexibility in operation.

Characteristics of railway electrification[edit]


Power is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of two forms. The first is an overhead line or catenary wire suspended from poles or towers along the track or from structure or tunnel ceilings. Locomotives or multiple units pick up power from the contact wire with pantographs on their roofs that press a conductive strip against it with a spring or air pressure. Examples are described later in this article.


In comparison to the principal alternative, the diesel engine, electric railways offer substantially better energy efficiency, lower emissions and lower operating costs. Electric locomotives are usually quieter, more powerful, and more responsive and reliable than diesels. They have no local emissions, an important advantage in tunnels and urban areas. Some electric traction systems provide 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, electricity is generated from diverse sources including many that do not produce carbon dioxide such as nuclear power and renewable forms including hydroelectric, geothermal, wind and solar.

Disadvantages of electric traction include high capital costs that may be uneconomic on lightly trafficked routes; a relative lack of flexibility since electric trains need electrified tracks or onboard supercapacitors and charging infrastructure at stations;[1] and a vulnerability to power interruptions. Different regions may use different supply voltages and frequencies, complicating through service. The limited clearances available under catenaries may preclude efficient double-stack container service. The lethal voltages on contact wires and third rails are a safety hazard to track workers, passengers and trespassers. Overhead wires are safer than third rails, but they are often considered unsightly.


Electrification systems in Europe:
  750 V DC
  1.5 kV DC
  3 kV DC
High speed lines in France, Spain, Italy, United Kingdom, the Netherlands, Belgium and Turkey operate under 25 kV, as high power lines in the former Soviet Union as well.

Electrification systems are classified by three main parameters:

Standardised voltages[edit]

Six of the most commonly used voltages have been selected for European and international standardisation. These are independent of the contact system used, so that, for example, 750 V DC may be used with either third rail or overhead lines (the latter normally by trams).

There are many other voltage systems used for railway electrification systems around the world, and the list of current systems for electric rail traction covers both standard voltage and non-standard voltage systems.


Electrification system Voltage
25 kV AC, 50 Hz 17.5 kV 19 kV 25 kV 27.5 kV 29 kV

Direct current[edit]

Railways must operate at variable speeds. Until recently this was only practical with the brush-type universal motor, which operates best on DC. For this reason, most early electrified railways used DC and many still do, particularly rapid transit (subways) and trams. Speed was controlled by connecting the traction motors in various series-parallel combinations and by inserting and removing starting resistances to limit motor current.

Motors have very little room for electrical insulation so they generally have low voltage ratings. Because transformers cannot step down DC voltages, the train must be supplied with a relatively low DC voltage that the motors can use directly. The most common DC voltages are listed in the previous section. Third (and fourth) rail systems almost always use voltages below 1 kV for safety reasons while overhead wires usually use higher voltages for efficiency. ("Low" voltage is relative; even 600 V can be instantly lethal when touched.)

Since utilities supply high voltage AC, DC railways use converter stations to produce low voltage DC. Originally they used rotary converters, a few of which are even still in operation, but most were supplanted first by mercury arc rectifiers and then by semiconductor rectifiers.

Because electrical power is equal to voltage times current, the relatively low voltages in a DC system imply relatively high currents. Minimizing resistive losses requires thick, short supply cables and closely spaced converter stations.


The 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.[4]

At the same voltage, DC often has less loss than AC, and for this reason High voltage direct current is already used on some bulk power transmission lines. DC avoids the electromagnetic radiation inherent with AC, and on a railway this also reduces interference with signalling and communications and mitigates hypothetical EMF risks. DC also avoids the power factor problems of AC.

Of particular interest to railroading is that DC can supply constant power with a single ungrounded wire. Constant power with AC requires three-phase transmission with at least two ungrounded wires. A few railroads have tried 3-phase but its substantial complexity has made single-phase standard practice despite the interruption in power flow that occurs twice every cycle.

An experimental 6kV DC railway was built in the Soviet Union.


Overhead systems[edit]

Main article: Overhead line
The Tyne and Wear Metro is the only United Kingdom system that uses 1,500 V DC.

1,500 V DC is used in the Netherlands, Japan, Hong Kong (parts), Republic of Ireland, Australia (parts), India (around the Mumbai area alone,[5] has been converted to 25 kV AC like the rest of the country[6]), France (also using 25 kV 50 Hz AC), New Zealand (Wellington) and the United States (Chicago area on the Metra Electric district and the South Shore Line interurban line). In Slovakia, there are two narrow-gauge lines in the High Tatras (one a cog railway). In Portugal, it is used in the Cascais Line and in Denmark on the suburban S-train system.



3 kV DC is used in Belgium, Italy, Spain, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia, South Africa and former Soviet Union countries (also using 25 kV 50 Hz AC). It was formerly used by the Milwaukee Road from Harlowton, Montana to Seattle-Tacoma, across the Continental Divide and including extensive branch and loop lines in Montana, and by the Delaware, Lackawanna & Western Railroad (now New Jersey Transit, converted to 25 kV AC) in the United States, and the Kolkata suburban railway (Bardhaman Main Line) in India, before it was converted to 25 kV 50 Hz AC.


Third rail[edit]

Main article: Third rail
A bottom-contact third rail on the Amsterdam Metro, the Netherlands

Most electrification systems use overhead wires, but third rail is an option up to about 1,200 V. Third rail systems exclusively use DC distribution. The use of AC is not feasible because the dimensions of a third rail are physically very large compared with the skin depth that the AC current penetrates to (0.3 millimetre in a steel rail). This effect makes the resistance per unit length unacceptably high compared with the use of DC.[7] Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.


DC systems (especially third rail systems) are limited to relatively low voltages and this can limit the size and speed of trains and cannot use low-level platform and also limit the amount of air-conditioning that the trains can provide. This may be a factor favouring overhead wires and high voltage AC, even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100 mph (160 km/h) because above that speed reliable contact between the shoe and the rail cannot be maintained.



A new approach to avoiding overhead wires is taken by the "second generation" tram/streetcar system in Bordeaux, France (entry into service of the first line in December 2003; original system discontinued in 1958) with its APS (alimentation par sol – ground current feed). This involves a third rail which is flush with the surface like the tops of the running rails. The circuit is divided into segments with each segment energized in turn by sensors from the car as it passes over it, the remainder of the third rail remaining "dead". Since each energized segment is completely covered by the lengthy articulated cars, and goes dead before being "uncovered" by the passage of the vehicle, there is no danger to pedestrians. This system has also been adopted in some sections of the new tram systems in Reims, France (opened 2011) and Angers, France (also opened 2011). Proposals are in place for a number of other new services including Dubai, UAE; Barcelona, Spain; Florence, Italy; Marseille, France; Gold Coast, Australia; Washington, D.C., U.S.A.; Brasília, Brazil and Tours, France. At least initially there were teething troubles in terms of maintaining current feed, however, and the fact that the system is used exclusively in the historic center, with the cars on leaving this zone converting to conventional overhead pickup, underlines how, esthetics aside, for streetcars/trams it is hard to beat the overhead wire system in terms of overall efficiency.

Fourth rail[edit]

Notice the unprotected third rail at the right of the track and the fourth rail at the center.
With top-contact third (and fourth) rail a heavy shoe suspended from a wooden beam attached to the bogies collects power by sliding over the top surface of the conductor rail.

The London Underground in England is one of the few networks that uses a four-rail system. 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 +420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary or a third rail.


London's sub-surface underground railways also operate on the four-rail scheme since in a number of areas (for example the Piccadilly Line and Metropolitan Line services to Uxbridge) sub-surface and deep-level stock run on the same tracks.




A few lines of the Paris Métro in France operate on a four-rail power scheme because they run on natural rubber tyres which run on a pair of narrow roadways made of steel and, in some places, concrete. Since the tyres do not conduct the return current, the two guide rails provided outside of the running 'roadways' double up as conductor rails, so at least electrically it fits as a four-rail scheme. One of the guide rails is bonded to the return conventional running rails situated inside the roadway so a single polarity supply is actually required. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey.[8] The loop was originally provided to save the original steam locomotives having to 'run around' the rest of the train saving much time. Today, the driver doesn't have to change ends at termini provided with such a loop, but the time saving is not so significant as it takes almost as long to drive round the loop as it does to change ends. Many of the original loops have been lost as lines were extended.

Alternating current[edit]

Railways and electrical utilities use AC for the same reason: to use transformers, which require AC, to produce higher voltages. Power is voltage times current, so the higher the voltage, the lower the current for the same power. Lower current means lower line loss and/or the ability to use lighter and less expensive conductors.

Because AC implies very high voltages, it is only used on overhead wires, never on third rails. Inside the locomotive, another transformer steps the voltage down for use by the traction motors and auxiliary loads.

Low-frequency alternating current[edit]

15 kV 16.7 Hz AC system used in Switzerland
The world's first AC locomotive in Valtellina (1898–1902). Power supply: 3-phase 15 Hz AC, 3000 V, (AC motor, 70 km/h). It was designed by Kálmán Kandó of Ganz Company, Hungary.

DC commutating electric motors are also known as universal motors because they can also operate on AC; reversing the current in both stator and rotor does not reverse the motor. But the now-standard AC distribution frequencies of 50 and 60 Hz caused difficulties with inductive reactance and eddy current losses, particularly in non-laminated pole pieces. Many railways chose low AC frequencies to overcome these problems. They must be converted from utility power by motor-generators or static inverters at the feeding substations or generated at dedicated traction powerstations.

These low frequencies were later made completely unnecessary by high power locomotive rectifiers that can convert any AC frequency to DC: first the mercury-arc rectifier and then the semiconductor rectifier. Some AC railways have been converted to standard grid frequencies but low frequencies are still widely used due to large sunken equipment costs.

Five European countries, namely, Germany, Austria, Switzerland, Norway and Sweden, standardized on 15 kV 16⅔ Hz (the 50 Hz mains frequency divided by three) single-phase AC. On 16 October 1995, Germany, Austria and Switzerland changed from 16⅔ Hz to 16.7 Hz which is no longer exactly 1/3 of the grid frequency. This solved overheating problems with the rotary converters used to generate some of this power from the grid supply.[9]




Polyphase alternating current systems[edit]

Train using a multiphase electrification system on the Petit train de la Rhune, France




The system is still used on four mountain railways, using 725 to 3000 V at 50 or 60 Hz: the (Corcovado Rack Railway in Rio de Janeiro, Brazil, Jungfraubahn and Gornergratbahn in Switzerland and the Petit train de la Rhune in France).

Standard frequency alternating current[edit]





A list of the countries using the 25 kV AC 50 Hz single-phase system can be found in the list of current systems for electric rail traction.

Close-up view of catenary on Northeast Corridor, United States




In Japanese Shinkansen lines, there are ground-operated switched sections installed instead of neutral sections. The sections detect trains running within the section and automatically switch the power supply in 0.3 s,[13] which eliminates the need to shut off power at any time.

World electrification[edit]

In 2006, 240,000 km (25% by length) of the world rail network was electrified and 50% of all rail transport was carried by electric traction.

Advantages and disadvantages[edit]


Newly electrified lines often show a "sparks effect", whereby electrification in passenger rail systems leads to significant jumps in patronage / revenue.[14] The reasons may include electric trains being seen as more modern and attractive to ride,[15][16] faster and smoother service,[14] 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.[14][15]


  • lower cost of building, running and maintaining locomotives and multiple units
  • higher power-to-weight ratio, resulting in
    • fewer locomotives
    • faster acceleration
    • higher practical limit of power
    • higher limit of speed
  • less noise pollution (quieter operation)
  • faster acceleration clears lines quicker to run more trains on the track in urban rail uses
  • 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


Disadvantages include:

Large cargo may require special cars
Most overhead electrifications do not allow sufficient clearance for a double-stack car.[citation needed]
  • Theft: the high scrap value of copper and the unguarded, remote installations make overhead cables an attractive target for scrap metal thieves.[18] Attempts at theft of live 25 kV cables may end in the thief's death from electrocution.[19] In the UK, cable theft is claimed to be one of the biggest sources of delay and disruption to train services.[20]


Maintenance costs of the lines may be increased, but many systems claim lower costs due to reduced wear-and-tear from lighter rolling stock.[21] 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.




Summary of advantages and disadvantages:

  • Lines with low frequency of traffic may not be feasible for electrification (especially using regenerative braking), because lower running cost of trains may be overcome by the higher costs of maintenance. Therefore most long-distance lines in North America and many developing countries are not electrified due to relatively low frequency of trains.

Energy efficiency[edit]



Central station electricity can often be generated with higher efficiency than a mobile engine/generator.[22]

Large fossil fuel power stations operate at high efficiency,[23][24] and can be used for district heating or to produce district cooling, leading to a higher total efficiency.

Energy sources unsuitable for mobile power plants, such as nuclear power, renewable hydroelectricity, or wind power can be used. According to widely accepted global energy reserve statistics,[25] the reserves of liquid fuel are much less than gas and coal (at 42, 167 and 416 years respectively). Most countries with large rail networks do not have significant oil reserves and those that did, like the United States and Britain, have exhausted much of their reserves and have suffered declining oil output for decades. Therefore, there is also a strong economic incentive to substitute other fuels for oil. Rail electrification is often considered an important route towards consumption pattern reform.[26][27] However, there are no reliable, peer-reviewed studies available to assist in rational public debate on this critical issue.

External cost[edit]

The external cost of railway is lower than other modes of transport but the electrification brings it down further if it is sustainable.

Also, the lower cost of energy from well to wheel and the ability to reduce pollution and greenhouse gas in the atmosphere according to the Kyoto Protocol is an advantage.



Non-contact systems[edit]

It is possible to supply power to an electric train by inductive coupling. This allows the use of a high-voltage, insulated, conductor rail. Such a system was patented in 1894 by Nikola Tesla, US Patent 514972.[28] It requires the use of high-frequency alternating current. Tesla did not specify a frequency but George Trinkaus[29] suggests that around 1,000 Hz would be likely.

Inductive coupling is widely used in low-power applications, such as re-chargeable electric toothbrushes. The contactless technology for rail vehicles is currently being marketed by Bombardier as PRIMOVE.[30]

See also[edit]


  1. ^ "Guangzhou supercapacitor tram unveiled". 2014-06-13. 
  3. ^ IEC 60850: Railway applications – Supply voltages of traction systems, 3rd edition (2007)
  4. ^ , P. Leandes and S. Ostlund. "A Concept for an HVDC Traction System" in "International Conference on Main Line Railway Electrification", Hessington, England, September 1989
  5. ^ [1]
  6. ^ [2]
  8. ^ For example: Line 6 has a reversing loop at Charles de Gaulle Étoile but not at Nation
  9. ^ Linder, C. (Munich 2002). "Umstellung der Sollfrequenz im zentralen Bahnstromnetz von 16 2/3 Hz auf 16,70 Hz" [Switching the frequency in train electric power supply network from 16 2/3 Hz to 16,70 Hz]. Elektrische Bahnen (in German) (Book 12) (Oldenbourg-Industrieverlag). ISSN 0013-5437. 
  10. ^ Southern Electric
  11. ^ History of Southern Electrification Part 1
  12. ^ History of Southern Electrification Part 2
  13. ^ Railway Technical Research Institute. "Concurrent-feeding power switching system for Shinkansen switching sections" (in Japanese). Retrieved 21 May 2011. 
  14. ^ a b c "Start Slow With Bullet Trains". Miller-McCune. 2 May 2011. Retrieved 27 February 2012. 
  15. ^ a b "Cumbernauld may be on track for railway line electrification". Cumbernauld News. 14 January 2009. Retrieved 27 February 2012. 
  16. ^ "Electric Idea". Bromsgrove Advertiser. 8 January 2008. Retrieved 27 February 2012. 
  17. ^ "Committee Meeting - Royal Meteorological Society - Spring 2009". Royal Meteorological Society (rmets.org). Retrieved 15 September 2012. 
  18. ^ "Network Rail - Cable Theft". Network Rail (www.networkrail.co.uk). Retrieved 15 September 2012. 
  19. ^ "Police probe cable theft death link". ITV News. 27 June 2012. Retrieved 15 September 2012. 
  20. ^ Sarah Saunders (28 June 2012). "Body discovery linked to rail cables theft". ITV News. Retrieved 7 May 2014. 
  22. ^ It turns our 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 diesel is based on the lower heat of combustion of fuel while power plants 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).
  23. ^ http://gigaom.com/cleantech/ge-to-crank-up-gas-power-plants-like-jet-engines/
  24. ^ http://www.ge-flexibility.com/solutions/flexefficiency-50-combined-cycle-power-plant/index.html
  25. ^ "Worldometers – real time world statistics". Worldometers. Retrieved 27 January 2011. 
  26. ^ "Supplication". Sahar English TV. Retrieved 27 January 2011. 
  27. ^ "Year of revision to pattern of consumption". The Office of the Supreme Leader, Sayyid Ali Khamenei. 20 March 2009. Retrieved 27 January 2011. 
  28. ^ http://www.google.com/patents?vid=514972
  29. ^ Trinkaus, George, Tesla, the lost inventions, pp 28–29, High Voltage Press, Portland, OR, 1988
  30. ^ http://www.bombardier.com/en/transportation/sustainability/technology/primove-catenary-free-operation


  • Дробинский В.А., Егунов П.М. "Как устроен и паботает тенловоз" (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).
  • Moody, G T (1960). "Part One". Southern Electric (3rd edition ed.). London: Ian Allan Ltd. 

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