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. 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.
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. 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. There used to be a historical concern for double-stack rail transport regarding clearances with overhead lines but it is no longer universally true as of 2022[update], with both Indian Railways and China Railway regularly operating electric double-stack cargo trains under overhead lines.
Railway electrification has constantly increased in the past decades, and as of 2012, electrified tracks account for nearly one-third of total tracks globally.
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.
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 and IEC 60850. These take into account the number of trains drawing current and their distance from the substation.
|Min. non-permanent||Min. permanent||Nominal||Max. permanent||Max. non-permanent|
|600 V DC||400 V||400 V||600 V||720 V||800 V|
|750 V DC||500 V||500 V||750 V||900 V||1,000 V|
|1,500 V DC||1,000 V||1,000 V||1,500 V||1,800 V||1,950 V|
|3 kV DC||2 kV||2 kV||3 kV||3.6 kV||3.9 kV|
|15 kV AC, 16.7 Hz||11 kV||12 kV||15 kV||17.25 kV||18 kV|
|25 kV AC, 50 Hz (EN 50163)
and 60 Hz (IEC 60850)
|17.5 kV||19 kV||25 kV||27.5 kV||29 kV|
Main article: Overhead line
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 3,000 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, trolleybus networks and underground (subway) systems as the traction motors accept this voltage without the weight of an on-board transformer.
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.
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.: 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.
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.
Main article: Third 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. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
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.
The Expo and Millennium Line of the Vancouver SkyTrain use side contact fourth-rail systems for their 650V 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 propulsion system used on the 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.
Main article: Rubber-tyred metro
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+1⁄2 in) standard gauge track between the roll ways operate in the same manner.
Railways and electrical utilities use AC as opposed to DC 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 (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.
As alternating current is used with high voltages, this method of electrification is only used on overhead lines, never on third rails for safety reasons. 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.
Five European countries – Germany, Austria, Switzerland, Norway and Sweden – have standardized on 15 kV 16+2⁄3 Hz (the 50 Hz mains frequency divided by three) single-phase AC. On 16 October 1995, Germany, Austria and Switzerland changed from 16+2⁄3 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.
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. 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.
Main article: 25 kV AC railway electrification
25 kV AC is used at 60 Hz on some US lines, in Saudi Arabia, 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 (including 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.
Main article: Three-phase AC railway electrification
Various railway electrification systems in the late nineteenth and twentieth centuries utilised 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.
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 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). 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. 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.: 3
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, diesel motors decrease in efficiency in non-nominal regimes at low power 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, and can be used for district heating or to produce district cooling, leading to a higher total efficiency.  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.
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 effects are a large factor with electrification. 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. 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. 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.
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. 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.
Newly electrified lines often show a "sparks effect", whereby electrification in passenger rail systems leads to significant jumps in patronage / revenue. The reasons may include electric trains being seen as more modern and attractive to ride, faster, quieter and smoother service, 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.
See also: 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 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 7.45 m (24.4 ft) to accommodate double-stack container trains without the need of well-wagons.
As of 2012, electrified tracks account for nearly one third of total tracks globally.
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+2⁄3 Hz and 20,440 km (12,700 mi) electrified at 1.5 kV DC.: 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. China has the largest electrified railway length with just over 70% of the network. 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 and Israel Railways.
The Trans-Siberian Railway mainly in Russia is completely electrified, making it one of the longest stretches of electrified railways in the world.
He was to produce the first motor that operated without gears of any sort, having its armature direct-connected to the car axle.