Third rail

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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 usually supplied with direct current.

Modern tram systems with street running avoid the electrical injury risk of the exposed electric rail by implementing a segmented ground-level power supply, where each segment is electrified only while covered by a vehicle which is using its power.^[1]

The third-rail system of electrification is not related to the third rail used in dual-gauge railways.

The system is generally associated with a low voltage (rarely above 750 V) and is far less used for main lines than overhead line, which with a higher voltage permit more distance between the substations. Also, for safety reasons, third-rail systems are generally fully grade separated. Third rail found its niche in metro systems, where a smaller tunnel is more important than having fewer substations. However, there are some main lines that use third rail, like lines in Southern England, Merseyrail, Long Island Rail Road, Hudson and Harlem lines of Metro North Railroad and Mitre, Sarmiento and Urquiza lines in Greater Buenos Aires.