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Third rail

From Academic Kids

"Third rail" is sometimes used as a metaphor in politics: see third rail (metaphor)
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Thirdrail.SouthStation.agr.jpg
Third rail at Boston's South Station

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, DC Metro systems, the San Francisco BART system, the Chicago 'L', most of the Metro-North Railroad and Long Island Rail Road in New York, the Toronto subway, and the MBTA in Boston. In the UK, third rails are used on the London Tube system (which uses a fourth rail as well), the suburban railway network in and around south London and south-east England and the Glasgow Subway system. Subway systems (U-bahnen) in Germany and the suburban trains (S-bahnen) in Hamburg and Berlin use a third rail.

This third-rail system is unrelated to the third rail used in dual-gauge railways.

Contents

History

Third-rail electric systems are, apart from on-board batteries, the oldest means of supplying electric power to trains. An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Exhibition of 1879. Third-rail systems began to be used in public transit in the 1880s for tram (or streetcar) systems and standard-gauge railways. A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890.

A widespread belief that African-American inventor Granville Woods invented the third rail is based on his Template:US patent, granted in 1901. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's Template:US patent of 1882.

Technical aspects

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 sliding "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 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V (suburban trains in Hamburg), and direct current (DC) is 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.

In line M1 of the Milan underground, the third rail is used as the return electrical line (with potential near the ground) and the live electrical connection is made with a sliding block on the side of the car contacting to a electrical bar located next to the railway (between the railway and the opposite direction railway) approximately 1 m (3') above the rail level. In this manner there are four rails. In the northern part of the line the more common overhead lines system is used.

One method for reducing current losses is to attach strips of aluminum (which is a better conductor than steel of electricity) to the steel third rail. Because aluminum has a different coefficient of thermal expansion from steel, the strips must be applied on both sides and riveted at frequent intervals. (The third rail in the photo above employs this system. Click on the photo to see it more clearly.)

Disadvantages of third rail

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. There are unverified reports that people have died as a result of urinating on the third rail, the urine stream completing an electrical circuit that results in the victim being electrocuted.
  • 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 have largely restricted their use to relatively low-speed, lightweight, trains of the type used in mass transit systems, although 750 V DC third rail is used on many hundreds of main line railway route miles across south and south-east England. Capacity is also limited by speed restrictions - 160 km/h (100 mph) is considered to be the maximum speed at which a contact shoe can reliably collect power from a third-rail system.
  • Infrastructure restrictions: Junctions and other pointwork make it necessary to leave gaps in the live rail at times, as do level crossings. This is not usually a problem as most third-rail rolling stock has multiple current collection shoes along the length of the train, but under certain circumstances it is possible for a train to become "gapped" - stalled with none of its shoes in contact with the live rail. When this happens it is usually necessary for the train to be shunted back onto a live section either by a rescue locomotive or another service train, although in some circumstances it is possible to use jumper cables to temporarily hook the train's current collectors to the nearest section of live rail. Especially given that gapping tends to happen at complex, important junctions, it can be a major source of disruption.

Advantages of third rail

However, third-rail systems are less expensive to install than overhead wire systems, less prone to weather damage (other than flooding and icing, which cause major problems), and better able to be fitted into small tunnels. One further argument cited in favour of third-rail systems is visual intrusion, since they don't need an overhead wire system which some people perceive as unsightly. 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.

Variants and alternatives

Surface current collection

Street-compatible third-rail systems are known as surface current collection. This variant of third rail power supply, used as far back as the 1880s, has recently made a comeback on a new tramway in Bordeaux.

Overhead lines

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, Metro-North's New Haven Division, Boston's Blue Line or Milan subway (line M1).

See also

External links

nl:Derde rail ja:第三軌条方式 zh:軌道供電 pl:trzecia szyna

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