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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 Los Angeles and Washington, D.C Metro systems, the San Francisco BART system, the Chicago 'El', and the large suburban 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. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side or bottom of the third rail, allowing the protective cover to be mounted directly to its top surface.

Whereas overhead-wire systems can operate at 25,000 volts or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 750 volts, and direct current (DC) is commonly used.

As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tires, as on part of the Paris Métro, a separate live rail must be provided for the return current; this third and fourth rail design has other advantages and a few steel-wheel systems also use it, the largest being the London Underground.

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

However, third-rail systems are less expensive to install than overhead wire systems, less prone to weather damage, and better able to be fitted into small tunnels. 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 and DC power, even where overhead lines would otherwise be practicable, due to the high cost of retrofitting.

See also Level crossing.

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Another use of the term third rail is to denote a political idea or topic that is so unpopular that a politician or public official who suggests it becomes the subject of public derision; for example, a politician who would advocate the repeal of the U.S. social security program. The analogy is that touching the third rail results in instant death.