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:
- 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 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 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 largely restricted their use to relatively low-speed, lightweight, trains of the type used in mass transit systems.
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.
External links
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.