Railway electrification system

Last updated

An unrefurbished Metro-Cammell train on the Kowloon-Canton Railway British Section in Hong Kong in 1993. The Kowloon-Canton Railway British Section is the oldest railway in Hong Kong. It started to operate in 1910 and connects to the Guangzhou-Shenzhen railway. KCRC Metro Cammell train, 1993.JPG
An unrefurbished Metro-Cammell train on the Kowloon-Canton Railway British Section in Hong Kong in 1993. The Kowloon-Canton Railway British Section is the oldest railway in Hong Kong. It started to operate in 1910 and connects to the Guangzhou-Shenzhen railway.
Transition zone of third-rail to overhead-wire supply on Chicago's Yellow Line (the "Skokie Swift"), shown shortly before the conversion to third rail operation in September 2004. 3rd rail to overhead wire transition zone on the Skokie Swift.jpg
Transition zone of third-rail to overhead-wire supply on Chicago's Yellow Line (the "Skokie Swift"), shown shortly before the conversion to third rail operation in September 2004.
An early rail electrification substation at Dartford Dartford Junction - geograph.org.uk - 198168.jpg
An early rail electrification substation at Dartford

A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), electric multiple units (passenger cars with their own motors) or both. Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network and distributed to the trains. Some electric railways have their own dedicated generating stations and transmission lines, but most purchase power from an electric utility. The railway usually provides its own distribution lines, switches, and transformers.


Power is supplied to moving trains with a (nearly) continuous 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, 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 engine, electric railways offer substantially better energy efficiency, lower emissions, and lower operating costs. Electric locomotives are also 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 products, electricity can be generated from diverse sources, including renewable energy. [1] Historically concerns of resource independence have played a role in the decision to electrify railway lines. The landlocked Swiss confederation which almost completely lacks oil or coal deposits but has plentiful hydropower electrified its network in part in reaction to supply issues during both World Wars. [2] [3]

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 third rails or overhead wires), and a vulnerability to power interruptions. [1] Electro-diesel locomotives and electro-diesel multiple units 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. The limited clearances available under overhead lines may preclude efficient double-stack container service. [1] However, Indian Railways [4] and China Railway [5] [6] [7] operate double-stack cargo trains under overhead wires with electric trains.

Railway electrification has constantly increased in the past decades, and as of 2012, electrified tracks account for nearly one-third of total tracks globally. [8]


Electrification systems in Europe:
.mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{}
750 V DC
1.5 kV DC
3 kV DC
15 kV AC
25 kV AC
High speed lines in France, Spain, Italy, United Kingdom, the Netherlands, Belgium and Turkey operate under 25 kV, as do high power lines in the former Soviet Union as well. Europe rail electrification en.svg
Electrification systems in Europe:
  750 V DC
  1.5 kV DC
  3 kV DC
   15 kV AC
   25 kV AC
High speed lines in France, Spain, Italy, United Kingdom, the Netherlands, Belgium and Turkey operate under 25 kV, as do high power lines in the former Soviet Union as well.

Electrification systems are classified by three main parameters:

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.

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 V 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 EN 50163 [9] and IEC 60850. [10] These take into account the number of trains drawing current and their distance from the substation.

Electrification systemVoltage
Min. non-permanentMin. permanentNominalMax. permanentMax. non-permanent
600 V DC 400 V400 V600 V720 V800 V
750 V DC500 V500 V750 V900 V1,000 V
1,500 V DC1,000 V1,000 V1,500 V1,800 V1,950 V
3 kV DC2 kV2 kV3 kV3.6 kV3.9 kV
15 kV AC, 16.7 Hz 11 kV12 kV15 kV17.25 kV18 kV
25 kV AC, 50 Hz (EN 50163)
and 60 Hz (IEC 60850)
17.5 kV19 kV25 kV27.5 kV29 kV

Direct current

Overhead systems

Electric locomotives under an 15 kV AC overhead line in Sweden Three engines of type Rc4.jpg
Electric locomotives under an 15 kV AC overhead line in Sweden
Nottingham Express Transit in the UK uses a 750 V DC overhead, in common with most modern tram systems. NET tram 201-03.jpg
Nottingham Express Transit in the UK uses a 750 V DC overhead, in common with most modern tram systems.

1,500 V DC is used in Japan, Indonesia, Hong Kong (parts), Ireland, Australia (parts), France (also using 25 kV 50 Hz AC), New Zealand (Wellington), Singapore (on the North East MRT Line), the United States (Chicago area on the Metra Electric district and the 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 kV on the HSL-Zuid and Betuwelijn, and 3000 V south of Maastricht. In Portugal, it is used in the Cascais Line and in Denmark on the suburban S-train system (1650 V DC).

In the United Kingdom, 1,500 V DC was used in 1954 for the 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 and Manchester, now converted to 25 kV AC. It is now only used for the Tyne and Wear Metro. In India, 1,500 V DC was the first electrification system launched in 1925 in Mumbai area. Between 2012 and 2016, the electrification was converted to 25 kV 50 Hz, which is the countrywide system.

3 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 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.

DC voltages between 600 V and 800 V are used by most tramways (streetcars), trolleybus networks and underground (subway) systems as the traction motors accept this voltage without the weight of an on-board transformer.

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. [11]

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. [12] :6–7 UIC conducted a case study for the conversion of the Bordeaux-Hendaye railway line (France), currently electrified at 1.5 kV DC, to 9 kV DC and found that the conversion would allow to use less bulky overhead wires (saving €20 million per 100 route-km) and lower the losses (saving 2 GWh per year per 100 route-km; equalling about €150,000 p.a.). The line chosen is one of the lines, totalling 6000 km, that are in need of renewal. [13]

In the 1960s the Soviets experimented with boosting the overhead voltage from 3 to 6 kV. DC rolling stock was equipped with ignitron-based converters to lower the supply voltage to 3 kV. The converters turned out to be unreliable and the experiment was curtailed. In 1970 experimental works on 12 kV DC system proved a.o. that the equivalent loss levels for a 25 kV AC system could be achieved with DC voltage between 11 and 16 kV. In the 1980s and 1990s experimental 12 kV DC system was being tested on the October Railway near Leningrad (now Petersburg). The experiments ended in 1995 due to the end of funding. [14]

Third rail

A bottom-contact third rail on the Amsterdam Metro, Netherlands Derde rail.jpg
A bottom-contact third rail on the Amsterdam Metro, Netherlands
With top-contact third (and fourth) rail a heavy shoe attached to the underside of a wooden beam which in turn is attached to the bogie, collects power by sliding over the top surface of the conductor rail. Top contact pickup shoe.jpg
With top-contact third (and fourth) rail a heavy shoe attached to the underside of a wooden beam which in turn is attached to the bogie, collects power by sliding over the top surface of the conductor rail.

Most electrification systems use overhead wires, but third rail is an option up to 1,500 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 depth that AC penetrates to 0.3 millimetres or 0.012 inches in a steel rail. This effect makes the resistance per unit length unacceptably high compared with the use of DC. [15] Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.

Fourth rail

London Underground track at Ealing Common on the District line, showing the third and fourth rails beside and between the running rails EalingCommon3.jpg
London Underground track at Ealing Common on the District line, showing the third and fourth rails beside and between the running rails
A train on Milan Metro's Line 1 showing the fourth-rail contact shoe. Milan M1 train fourth-rail contact shoe.jpg
A train on Milan Metro's Line 1 showing the fourth-rail contact shoe.

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.

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 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 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.

Rubber-tyred systems

The bogie of an MP 05, showing the flanged steel wheel inside the rubber-tyred one, as well as the vertical contact shoe on top of the steel rail Atelier Fontenay - MP 05 - Bogie avant.jpg
The bogie of an MP 05, showing the flanged steel wheel inside the rubber-tyred one, as well as the vertical contact shoe on top of the steel rail
Bogie from an MP 89 Paris Metro vehicle. The lateral contact shoe is located between the rubber tyres Bogie-metro-Meteor-p1010692.jpg
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 move on rubber tyres which roll on a pair of narrow roll ways made of steel and, in some places, of concrete. Since the tyres do not conduct the return current, the two guide bars provided outside the running 'roll ways' become, in a sense, a third and fourth rail which each provide 750 V DC, so at least electrically it is a four-rail system. Each 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 wagon, is effected by one contact shoe each that slide on top of each one of the running rails. This and all other rubber-tyred metros that have a 1,435 mm (4 ft 8+12 in) standard gauge track between the roll ways operate in the same manner. [16] [17]

Alternating current

Image of a sign for high voltage above a railway electrification system Railway electrification system.jpg
Image of a sign for high voltage above a railway electrification system

Railways and electrical utilities use AC for the same reason: to use transformers, which require AC, to produce higher voltages. The higher the voltage, the lower the current for the same power, which reduces line loss, thus allowing higher power to be delivered.

Because alternating current is used with high voltages, this method of electrification is only used on overhead lines, never on third rails. 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. 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

15 kV 16.7 Hz AC system used in Switzerland SBB-CFF-FFS Ae 6-6.jpg
15 kV 16.7 Hz AC system used in Switzerland

Five European countries, Germany, Austria, Switzerland, Norway and Sweden, have standardized on 15 kV 16+23 Hz (the 50 Hz mains frequency divided by three) single-phase AC. On 16 October 1995, Germany, Austria and Switzerland changed from 16+23 Hz to 16.7 Hz which is no longer exactly one-third of the grid frequency. This solved overheating problems with the rotary converters used to generate some of this power from the grid supply. [18]

In the US, the New York, New Haven, and Hartford Railroad, the Pennsylvania Railroad and the Philadelphia and Reading Railway adopted 11 kV 25 Hz single-phase AC. Parts of the original electrified network still operate at 25 Hz, with voltage boosted to 12 kV, while others were converted to 12.5 or 25 kV 60 Hz.

In the UK, the London, Brighton and South Coast Railway pioneered overhead electrification of its suburban lines in London, London Bridge to Victoria being opened to traffic on 1 December 1909. Victoria to Crystal Palace via Balham and West Norwood opened in May 1911. Peckham Rye to 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 serving Coulsdon North and Sutton railway station. [19] [20] The lines were electrified at 6.7 kV 25 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

25 kV AC is used at 60 Hz on some US lines, in western Japan, South Korea and Taiwan; and at 50 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 (incl. countries that still run conventional railways under DC but not in countries using 16.7 Hz, see above). On "French system" HSLs, the overhead line and a "sleeper" feeder line each carry 25 kV in relation to the rails, but in opposite phase so they are at 50 kV from each other; autotransformers equalize the tension at regular intervals.


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, transformed and 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 3-phase AC motors which require further conversion of the DC to 3-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). [21] 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. [22] 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 (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 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. [12] :3

Electric versus diesel

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. Thomas Robert Way00.jpg
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 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.

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, [23] diesel motors decrease in efficiency in non-nominal regimes at low power [24] 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 stations operate at high efficiency, [25] [26] and can be used for district heating or to produce district cooling, leading to a higher total efficiency.

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 trains 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 effects are a large factor with electrification.[ citation needed ] 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 ] If through traffic is to have any benefit, time-consuming engine switches must occur to make such connections or expensive 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. Power gaps can be overcome by on-board batteries or motor-flywheel-generator systems.[ citation needed ] In 2014, progress is being made in the use of large capacitors to power electric vehicles between stations, and so avoid the need for overhead wires between those stations. [27]

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. [28] 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. [29] The reasons may include electric trains being seen as more modern and attractive to ride, [30] [31] faster and smoother service, [29] 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. [29] [30]

Double-stack rail transport

Due to the 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 Africa 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 7.45 metres to accommodate double-stack container trains without the need of well-wagons.


  • No exposure to passengers to exhaust from the locomotive
  • Lower cost of building, running and maintaining locomotives and multiple units
  • 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)
  • Faster acceleration clears lines more quickly 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
  • More comfortable ride on multiple units as trains have no underfloor diesel engines
  • Somewhat higher energy efficiency [32] in part due to regenerative braking and less power lost when "idling"
  • More flexible primary energy source: can use coal, nuclear or renewable energy (hydro, solar, wind) as the primary energy source instead of diesel oil
  • 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.


The Royal Border Bridge in England, a protected monument. Adding electric catenary to older structures may be an expensive cost of electrification projects Berwick-upon-Tweed MMB 14 Royal Border Bridge.jpg
The Royal Border Bridge in England, a protected monument. Adding electric catenary to older structures may be an expensive cost of electrification projects
Most overhead electrifications do not allow sufficient clearance for a double-stack car. Each container may be 9 ft 6 in (2.90 m) tall and the bottom of the well is 1 ft 2 in (0.36 m) above rail, making the overall height 20 ft 2 in (6.15 m) including the well car. DTTX 724681 20050529 IL Rochelle.jpg
Most overhead electrifications do not allow sufficient clearance for a double-stack car. Each container may be 9 ft 6 in (2.90 m) tall and the bottom of the well is 1 ft 2 in (0.36 m) above rail, making the overall height 20 ft 2 in (6.15 m) including the well car.
  • 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 obstructions have to be altered for 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 may require line closures while the 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 of a moving train to become entangled with the 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. [34]
  • Theft: the high scrap value of copper and the unguarded, remote installations make overhead cables an attractive target for scrap metal thieves. [35] Attempts at theft of live 25 kV cables may end in the thief's death from electrocution. [36] 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. [37]
  • 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 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 predators, [38] 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. It is extremely costly to upgrade electrified lines to the correct clearances (21 ft 8 in or 6.60 m) to take double-stacked container trains.

World electrification

As of 2012, electrified tracks account for nearly one third of total tracks globally. [8]

As of 2018, there were 72,110 km (44,810 mi) of railways electrified at 25 kV, either 50 or 60 Hz; 68,890 km (42,810 mi) electrified at 3 kV DC; 32,940 km (20,470 mi) electrified at 15 kV 16.7 or 16+23 Hz and 20,440 km (12,700 mi) electrified at 1.5 kV DC. [12] :2

The Swiss rail network is the largest fully electrified network in the world and one of only two to achieve this, the other being Armenia. India and China have the largest electrified railway length with just over 70% of the network. [39] A number of countries have zero electrification length.

Several countries have announced plans to electrify all or most of their railway network such as Indian Railways, Israel Railways and Nederlandse Spoorwegen. In the Netherlands, the only not yet electrified lines are secondary freight-only lines, including the section of the Iron Rhine between Hamont (Belgium) and Weert, used by at least one train per week to keep it officially open, and the Zeelandic Flanders lines linking Terneuzen and Axelse Vlakte with Ghent, which are connected to Belgium but not to the rest of the Dutch network.

See also

Related Research Articles

Locomotive Self-propelled railway vehicle

A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight.

An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, trolleybuses or trams. It is known variously as:

Third rail Method of providing electric power to a railway train

A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from the outside environment. Third rail systems are often supplied from direct current electricity.

Electric locomotive Locomotive powered by electricity

An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or a supercapacitor.

A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, elevators or electric multiple unit.

Traction power network

A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses alternating current (AC) with a frequency lower than that of the national grid, such as in Germany, Austria and Switzerland.

25 kV AC railway electrification Standard current and voltage settings for most high-speed rail

Railway electrification systems using alternating current (AC) at 25 kilovolts (kV) are used worldwide, especially for high-speed rail.

15 kV AC railway electrification Standard current and voltage settings for much of Central Europes train transport

Railway electrification systems using alternating current (AC) at 15 kilovolts (kV) and 16.7 Hertz (Hz) are used on transport railways in Germany, Austria, Switzerland, Sweden, and Norway. The high voltage enables high power transmission with the lower frequency reducing the losses of the traction motors that were available at the beginning of the 20th century. Railway electrification in late 20th century tends to use 25 kV, 50 Hz AC systems which has become the preferred standard for new railway electrifications but extensions of the existing 15 kV networks are not completely unlikely. In particular, the Gotthard Base Tunnel still uses 15 kV, 16.7 Hz electrification.

Head-end power Electric power supply to trains by locomotives

In rail transport, head-end power (HEP), also known as electric train supply (ETS), is the electrical power distribution system on a passenger train. The power source, usually a locomotive at the front or 'head' of a train, provides the electricity used for heating, lighting, electrical and other 'hotel' needs. The maritime equivalent is hotel electric power. A successful attempt by the London, Brighton and South Coast Railway in October 1881 to light the passenger cars on the London to Brighton route heralded the beginning of using electricity to light trains in the world.

Electro-diesel locomotive Railway locomotive capable of running either under electrical or diesel power

An electro-diesel locomotive is a type of locomotive that can be powered either from an electricity supply or by using the onboard diesel engine. For the most part, these locomotives are built to serve regional, niche markets with a very specific purpose.

Rotary phase converter

A rotary phase converter, abbreviated RPC, is an electrical machine that converts power from one polyphase system to another, converting through rotary motion. Typically, single-phase electric power is used to produce three-phase electric power locally to run three-phase loads in premises where only single-phase is available.

Railroad electrification in the United States began at the turn of the 20th century and comprised many different systems in many different geographical areas, few of which were connected. Despite this situation, these systems shared a small number of common reasons for electrification.

Railway electric traction describes the various types of locomotive and multiple units that are used on electrification systems around the world.

Amtraks 25 Hz traction power system

Amtrak's 25 Hz traction power system is a traction power grid operated by Amtrak along the southern portion of its Northeast Corridor (NEC): the 225 route miles (362 km) between Washington, D.C. and New York City and the 104 route miles (167 km) between Philadelphia and Harrisburg, Pennsylvania. The Pennsylvania Railroad constructed it between 1915 and 1938. Amtrak inherited the system from Penn Central, the successor to Pennsylvania Railroad, in 1976 along with the Northeast Corridor. This is the reason for using 25 Hz, as opposed to 60 Hz, which is the standard for power transmission in North America. In addition to serving the NEC, the system provides power to NJ Transit Rail Operations (NJT), the Southeastern Pennsylvania Transportation Authority (SEPTA) and the Maryland Area Regional Commuter Train (MARC). Only about half of the system's electrical capacity is used by Amtrak. The remainder is sold to the commuter railroads who operate their trains along the corridor.

Multi-system (rail)

A multi-system locomotive, also known as a multi-system electric locomotive, multi-system electric multiple unit, or multi-system train, is an electric locomotive which can operate using more than one railway electrification system. Multi-system trains provide continuous journeys over routes which are electrified using more than one system.

Electrification of the New York, New Haven, and Hartford Railroad

The New York, New Haven and Hartford Railroad pioneered electrification of main line railroads using high-voltage, alternating current, single-phase overhead catenary. It electrified its mainline between Stamford, Connecticut, and Woodlawn, New York, in 1907, and extended the electrification to New Haven, Connecticut, in 1914. While single-phase AC railroad electrification has become commonplace, the New Haven's system was unprecedented at the time of construction. The significance of this electrification was recognized in 1982 by its designation as a National Historic Engineering Landmark by the American Society of Mechanical Engineers (ASME).

Railway electrification in Norway

The Norwegian railway network consists of 2,552 kilometers (1,586 mi) of electrified railway lines, constituting 62% of the Norwegian National Rail Administration's 4,114 kilometers (2,556 mi) of line. In 2008, electric traction accounted for 90% of the passenger kilometers, 93% of the tonne kilometers and 74% of the energy consumption of all trains running in Norway, with the rest being accounted for by diesel traction.

Railway electrification in the Soviet Union

While the former Soviet Union got a late start with rail electrification in the 1930s it eventually became the world leader in electrification in terms of the volume of traffic under the wires. During its last 30 years the Soviet Union hauled about as much rail freight as all the other countries in the world combined and in the end, over 60% of this was by electric locomotives. Electrification was cost effective due to the very high density of traffic and was at times projected to yield at least a 10% return on electrification investment. By 1990, the electrification was about half 3 kV DC and half 25 kV AC 50 Hz and 70% of rail passenger-km was by electric railways.

South African Class 20E

The Transnet Freight Rail Class 20E of 2013 is a South African electric locomotive.

Railway electrification in New Zealand consists of three separate electric systems, all in the North Island. Electrification was initially adopted by the New Zealand Railways for long tunnels; the Otira Tunnel, the Lyttelton Rail Tunnel and the two Tawa Tunnels of the Tawa Flat Deviation. Electrification of Wellington suburban services started with the Johnsonville Line and Kapiti Line out of Wellington from the 1930s. Auckland suburban services were electrified in 2014–2015. Electrification of long-distance services on the North Island Main Trunk (NIMT) dates from 1986. New long tunnels, for example the Rimutaka Tunnel and the Kaimai Tunnel, were operated by diesels, and the Otira and Lyttelton Tunnels have converted to diesel operation.


  1. 1 2 3 P. M. Kalla-Bishop, Future Railways and Guided Transport, IPC Transport Press Ltd. 1972, pp. 8-33
  2. "A train ride through history". SWI swissinfo.ch.
  3. "A nation of railway enthusiasts: a history of the Swiss railways". House of Switzerland.
  4. "Indian Railways sets new benchmark! Runs 1st Double-stack container train in high rise OHE electrified sections". 12 June 2020.
  5. "非人狂想屋 | 你的火车发源地 » HXD1B牵引双层集装箱列车" (in Chinese). Retrieved 1 July 2020.
  6. "Spotlight on double-stack container movement". @businessline. Retrieved 1 July 2020.
  7. "Aerodynamic Effects Caused by Trains Entering Tunnels". ResearchGate. Retrieved 1 July 2020.
  8. 1 2 "Railway Handbook 2015" (PDF). International Energy Agency. p. 18. Retrieved 4 August 2017.
  9. EN 50163: Railway applications. Supply voltages of traction systems (2007)
  10. IEC 60850: Railway applications – Supply voltages of traction systems, 3rd edition (2007)
  11. 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 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 kV).
  12. 1 2 3 Simiyu, Patrobers; Davidson, I.E. (2021). "MVDC Railway Traction Power Systems; State-of-the Art, Opportunities, and Challenges". Energies. MDPI. 14 (14). doi: 10.3390/en14144156 . ISSN   1996-1073.
  13. Future DC railway electrification system Go for 9kV (PDF). UIC Workshop on Energy Efficiency. Rotterdam: UIC. 11 September 2019.
  14. Аржанников, Б.А.; Галкин, А.Г.; Бурков, А.Т.; Мансуров, В.А.; Набойченко, И.О., Перспектива разработки системы электрической тяги постоянного тока повышенного напряжения 12, 24 кВ для скоростной магистрали Москва – Екатеринбург [Perspective of developing 12 or 24 kV DC electrification system for Moscow-Ekaterinburg high-speed line] (in Russian)
  15. 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
  16. "[MétroPole] De la centrale électrique au rail de traction". 10 August 2004. Archived from the original on 10 August 2004.
  17. Dery, Bernard. "Truck (bogie) - Visual Dictionary". www.infovisual.info.
  18. Linder, C. (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). Oldenbourg-Industrieverlag. ISSN   0013-5437.
  19. History of Southern Electrification Part 1
  20. History of Southern Electrification Part 2
  21. 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.96-7, ohmic losses pp.97-9
  22. Сидоров 1988 pp. 103-4, Сидоров 1980 pp. 122-3
  23. 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).
  24. 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
  25. Wang, Ucilia (25 May 2011). "Gigaom GE to Crank Up Gas Power Plants Like Jet Engines". Gigaom.com. Retrieved 4 February 2016.
  26. FlexEfficiency* 50 Combined Cycle Power Plant Archived 24 August 2012 at the Wayback Machine
  27. Railway Gazette International Oct 2014.
  28. "UK Network Rail electrification strategy report" Table 3.3, page 31. Retrieved on 4 May 2010
  29. 1 2 3 "Start Slow With Bullet Trains". Miller-McCune. 2 May 2011. Archived from the original on 28 January 2012. Retrieved 27 February 2012.
  30. 1 2 "Cumbernauld may be on track for railway line electrification". Cumbernauld News. 14 January 2009. Retrieved 27 February 2012.
  31. "Electric Idea". Bromsgrove Advertiser. 8 January 2008. Retrieved 27 February 2012.
  32. 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.
  33. AAR Plate H
  34. "Committee Meeting - Royal Meteorological Society - Spring 2009" (PDF). Royal Meteorological Society (rmets.org). Retrieved 15 September 2012.
  35. "Network Rail - Cable Theft". Network Rail (www.networkrail.co.uk). Retrieved 15 September 2012.
  36. "Police probe cable theft death link". ITV News. 27 June 2012. Retrieved 15 September 2012.
  37. Sarah Saunders (28 June 2012). "Body discovery linked to rail cables theft". ITV News. Retrieved 7 May 2014.
  38. Nachmann, Lars. "Tiere & Pflanzen Vögel Gefährdungen Stromtod Mehr aus dieser Rubrik Vorlesen Die tödliche Gefahr". Naturschutzbund (in German). Berlin, Germany. Retrieved 20 July 2016.
  39. "2019 年铁道统计公报" (PDF).