Double electrification
Encyclopedia
Double electrification is a system whereby a railway line is supplied power both via overhead catenary and a third rail
Third rail
A third rail is a method of providing electric power to a railway 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...

. This is done to enable trains that use either system of power to share the same railway line, for example in the case of mainline and suburban trains (as used at Hamburg
Hamburg
-History:The first historic name for the city was, according to Claudius Ptolemy's reports, Treva.But the city takes its modern name, Hamburg, from the first permanent building on the site, a castle whose construction was ordered by the Emperor Charlemagne in AD 808...

 S-Bahn
S-Bahn
S-Bahn refers to an often combined city center and suburban railway system metro in Austria, Germany, Switzerland and Denmark...

 between 1940 and 1955). Parts of the UK North London Line
North London Line
The North London Line is a railway line which passes through the inner suburbs of north London, England. Its route is a rough semicircle from the south west to the north east, avoiding central London. The line is owned and maintained by Network Rail...

, New York's Penn Station
Pennsylvania Station (New York City)
Pennsylvania Station—commonly known as Penn Station—is the major intercity train station and a major commuter rail hub in New York City. It is one of the busiest rail stations in the world, and a hub for inbound and outbound railroad traffic in New York City. The New York City Subway system also...

 and the adjoining East River Tunnels
East River Tunnels
The East River Tunnels are 4 single-track railroad tunnels that extend from the eastern end of Pennsylvania Station under 32nd and 33rd Streets in Manhattan and cross the East River to Long Island City in Queens. The tracks carry Long Island Rail Road and Amtrak trains travelling to and from Penn...

 and North River Tunnels are double-electrified.

The system is usually used only in exceptional cases as it can lead to problems caused by the interaction of the electric circuits; for example, where one system is powered with direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 and another by alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC), premagnetisation of the substation transformers of the AC system can occur.

A similar arrangement to double electrification is one in which both means of powering a train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

 are present, but not live simultaneously. Such arrangements can be found in frontier stations and in sections of railway used for running tests.

The building of the Channel Tunnel
Channel Tunnel
The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...

 between England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

  required triple electrification of the motive power, as on the Type 92 locomotive
British Rail Class 92
The British Rail Class 92 is a dual-voltage electric locomotive which can run on 25 kV AC from overhead wires or 750 V DC from a third rail. It was designed specifically to operate services through the Channel Tunnel between Britain and France...

 and British Rail Class 373
British Rail Class 373
The British Rail Class 373 or TGV-TMST train is an electric multiple unit that operates Eurostar's high-speed rail service between Britain, France and Belgium via the Channel Tunnel...

 Eurostar sets, because the French side uses overhead AC power, Belgium overhead DC, and the original route through Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...

used DC third-rail distribution.
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