Third rail

A third rail is a method of providing electricity to power a railroad, typically a mass transit system. Well-known examples of rail transit systems utilizing a third rail include the New York City subway system, the London Underground, the Los Angeles and Washington, D.C Metro systems, the San Francisco BART system, the Chicago 'El', and the large sububan railway network in and around London and south-east England.

Third rail electric systems are the oldest method of supplying power to trains. Experimental third rail systems were used as far back as the 1880s for tram (or streetcar) systems.

Another method of powering electric trains is the use of electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. On some metro/light-rail lines part of the line has a third rail, and another part overhead wires, and vehicles allow both, e.g. in Rotterdam.

The third rail is located either in between the two running rails, or by the side of them. The electricity is transmitted to the train by means of a "shoe" which contacts the rail. Third rail systems always use Direct current (DC) instead of Alternating current (AC) and run at a relatively low voltage (usually around 750 volts (DC)) in comparison to overhead wire systems (which can operate up to 25,000 volts (AC)).

Third rail systems have a number of significant problems and disadvantages, including:

Safety: Having an unguarded electrified rail is a major safety hazard, and many people have been killed by touching the rail or by stepping on it while attempting to cross the tracks. It is a verified urban legend that people (men) have died from urinating on the third rail, the urine stream completing an electrical circuit that results in the victim's being electrocuted. This myth probably got its start as as a result of the Lee V. Chicago Transit Authority case (the usually cited "evidence") involving a suit raised by the family of a man from Chicago's Chinatown who could not read the warning signs as they were not written in Mandarin and was electrocuted, having touched the third rail after stepping out onto the El tracks to urinate.
Limited capacity: A relatively low voltage is necessary in a third rail system, otherwise electricity would arc from the rail. This low voltage means that electrical feeder or sub-stations have to be set up at frequent intervals along the line in order to feed electricity into the system. This increases the cost of operating the railway. The low voltage also means that the system is prone to overload; this makes third rail systems unsuitable for trains demanding high amounts of power such as freight trains or high speed trains. These inherent limitations of third rail systems has restricted their use to relatively low-speed, lightweight, trains of the type used in mass transit systems.

Third rail systems are less expensive to install than are overhead wire systems. While sometimes used in new transit system construction, third rails are now considerably less popular than are overhead systems. Many older railways still use third rails, even where overhead lines would otherwise be practicable, due to the high cost of retrofitting.