Railway electric traction
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Railway electric traction describes the various types of locomotive and multiple units that are used on electrification systems around the world.
Railway electrification as a means of traction emerged at the end of the nineteenth century, although experiments in electric rail have been traced back to the mid-nineteenth century. Thomas Davenport, in Brandon, Vermont, erected a circular model railroad on which ran battery-powered locomotives (or locomotives running on battery-powered rails) in 1834. Robert Davidson, of Aberdeen, Scotland, created an electric locomotive in 1839 and ran it on the Edinburgh-Glasgow railway at 4 miles per hour. The earliest electric locomotives tended to be battery-powered. In 1880, Thomas Edison built a small electrical railway, using a dynamo as the motor and the rails as the current-carrying medium. The electric current flowed through the metal rim of otherwise wooden wheels, being picked up via contact brushes.
Electrical traction offered several benefits over the then predominant steam traction, particularly in respect of its quick acceleration (ideal for urban (metro) and suburban (commuter) services) and power (ideal for heavy freight trains through mountainous/hilly sections). A plethora of systems emerged in the first twenty years of the twentieth century.
Unit types 
DC traction units 
DC traction units use direct current drawn from either a conductor rail or an overhead line..
AC traction units 
Apart from a few cases, almost all AC Traction units draw alternating current from an overhead line.
Multi-system units 
Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way to accomplish this is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another and so the train arrives with one locomotive and then departs with another. Often, however, this is inconvenient and time-consuming. The switching stations have very sophisticated components and they are very expensive.
Another way is to use multi-system locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (1.5 kV DC, 3 kV DC, 15 kV 16⅔ Hz AC, 25 kV, 50 Hz AC). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train coasts for a short time.
Eurostar trains through the Channel Tunnel are multisystem; a significant part of the route near London is on southern England's 750 V DC third rail system, the route into Brussels is 3000 V DC overhead, while the rest of the route is 25 kV 50 Hz overhead. The need for these trains to use third rail ended upon completion of High Speed 1 in 2007. Southern England has some overhead/third rail dual-system locomotives and multiple units to allow through running between 750 V DC third rail south of London and the 25 kV AC overhead north and east of London.
Electro-diesel locomotives which can operate as an electric locomotive on electrified lines but have an on-board diesel engine for non-electrified sections or sidings have been used in several countries.
Czech Republic and Slovakia 
In the Czech Republic and Slovakia, the railways have both 3,000 V DC and 25 kV AC systems but there are no switching stations - the two systems meet at breaks on overhead wires. Only two of the breaks (Kutná Hora and Nedakonice) are in stations.
United Kingdom 
Electrification in the UK began in a piecemeal fashion. The earliest main line (as opposed to metro and tramway) systems were divided between low voltage third rail (commonly about 600 V DC) and overhead systems (a variety of voltages, both DC and AC were used). The third rail systems of this period eventually gave rise to the 750 V DC system in the southern part of the UK and a separate area with the same system around Merseyside.
Cheap loans to stimulate economic development in the 1930s gave rise to the several schemes of 1500 V DC electrification, mostly completed post war, notably between Liverpool Street and Shenfield, and the Woodhead Line. Starting with the West Coast Mainline electrification in the 1960s, the 25 kV AC overhead system was adopted for all subsequent mainline electrification in the UK (except for extensions to other existing systems, mostly on the southern third rail network).
In some areas with restricted clearances, particularly in urban areas in east London (converted from 1500 V DC) and on suburban routes around Glasgow, 6.25 kV was used. A system known as "Automatic Power Control" was developed to allow trains to automatically switch between the voltages whilst moving. All the driver had to do was shut off power and coast until clear of the neutral section; the system automatically opened the circuit breaker, detected a change in voltage and switched over the transformer to the correct input voltage setting, then closed the circuit breaker. This system proved somewhat unreliable and, with experience, it was found that less clearance was needed for 25 kV than had initially been allowed for. This allowed the 6.25 kV sections to be converted to 25 kV, with the last section, at the London end of the London Tilbury and Southend line, being converted in 1983.
United States 
In the United States, New Jersey Transit uses multisystem ALP-44 and ALP-46 locomotives for its Midtown Direct service into New York and Amtrak uses multi-system AEM-7, HHP-8 and Acela locomotives on the Northeast Corridor between Washington DC and Boston. In both cases, through trains run on both newer, 25 kV 60 Hz built or refurbished by their respective agencies since the 1980s and older, 12 kV 25 Hz inherited from the now-defunct Pennsylvania Railroad. The latter dates to the 1930s, when the Pennsylvania upgraded its electrified network from 650 V DC third rail.
Italian railways have two systems with overhead supply from a catenary: 3 kV DC and 25 kV AC. The 25 kV AC system is used on the new High speed lines.
Spanish railways have two systems with overhead supply from a catenary: 3 kV DC and 25 kV AC. The 25 kV AC system is used on the High speed lines.
In India 1500 V DC and 25 kV AC, 50 Hz, is used for main line trains.
The 1500 V DC overhead system (negative earth, positive catenary) is used around Mumbai. The Mumbai region is the last bastion of 1500 V DC electrified lines on Indian Railways. The Western Railway route from Mumbai Central is now completely changed to 25Kv AC. The 25 kV AC system with overhead lines is used throughout the rest of the country. The dual-voltage WCAM series locomotives haul intercity trains out of Mumbai DC suburban region. The new AC/DC EMU rakes used in Mumbai are also designed to operate with both DC and AC traction as the Mumbai area switches over to the 25 kV AC system which has provided access to the locomotives having AC Traction motors to serve in the suburbs.
The Kolkata Metro uses 750 V DC traction with a third rail for delivering the electricity to the EMUs. The Kolkata trams use 550 V DC with overhead lines with underground conductors. The catenary is at a negative potential.
South Africa 
South Africa has 15 km of dual system track, both 3 kV DC and 25 kV AC.
Battery electric rail vehicles 
A few battery electric railcars and locomotives were used in the twentieth century, but generally the use of battery power was not practical except in underground mining systems, see Railcar and Locomotive sections.
High-speed rail 
See also 
- J Halpin
- "Traxx locomotive family meets European needs". Railway Gazette International. 2008-01-07. Retrieved 2011-01-01. "Traxx MS (multi-system) for operation on both AC (15 and 25 kV) and DC (1·5 and 3 kV) networks"
- Railway Technical Web Page
- Short account of electric traction history up to the 1880s, with emphasis on Thomas Edison's experiments