Third rail

A third rail, also known as a conductor rail, electric rail, live rail, or power 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. 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, some main lines 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.