Third rail

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For the Squeeze single, see Third Rail (song). "Third rail" is sometimes used as a metaphor in politics: see third rail (metaphor). A third rail can also be part of a dual gauge setup.

A third rail is a method of providing electricity to power a railway, typically a mass transit or rapid transit system. Well-known examples of rail transit systems in North America utilizing a third rail include the metro systems of the New York City, Los Angeles, San Francisco, Chicago, Toronto and the Boston. In the UK, third rails are used on the London Underground system (which uses a fourth rail as well), the suburban railway network in and around South London, long-distance services across the south of England, the Glasgow Subway and on the Merseyrail network. German metro systems (U-Bahnen) and suburban trains in Hamburg and Berlin (S-Bahnen) use a third rail. The metro systems of Amsterdam, Netherlands, Warsaw in Poland, Moscow and St. Petersburg, Russia also use third rails to power their trains, as do parts of the Paris Métro. Parts of the Mexico City system are a rubber-tired metro, as are parts of the Paris Paris Métro, parts of the Santiago Metro in Chile and all of the Montreal Metro.

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

1 History
2 Technical aspects
3 Advantages of third rail
4 Disadvantages of third rail
5 Compromise systems
6 See also
7 External links

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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.

In 1901, Granville Woods, a prominent African-American inventor, was granted a patent Template:US patent, covering various proposed improvements to third rail systems. This has been cited to claim that he invented the third rail system of current distribution. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's Template:US patent of 1882, and third rails had been in successful use for over a decade, in installations including London and Brooklyn, New York. To what extent Woods' ideas were adopted is a matter of controversy.[1]

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Technical aspects

Image:Top contact pickup shoe.jpg

Image:Arcing pickup shoe.jpg

The third rail is located either in between the two running rails or, usually, on the outside of them. The electricity is transmitted to the train by means of a sliding "shoe" (pick-up or contact shoe) which is held in contact with 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 (side running) or bottom (bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as "top running". When the shoe slides on the bottom it is not affected by the build-up of snow or leaves.

The third rail is an alternative to electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. On some metro/light rail lines as well as regional rail lines, part of the line has a third rail and another part overhead wires, and vehicles allow both, e.g. in Rotterdam, Boston's Blue Line Milan subway (line M1) or Metro-North's New Haven Division (Commuter rail in North America). 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 parts of the Paris Métro, Mexico City Metro and Santiago Metro, as well as on all of the Montréal Métro, live guide bars must be provided to feed the current The return is effected through the rails of the conventional track between these guide bars, see rubber-tired metro. A third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails) design, that has other advantages, is used by a few steel-wheel systems. The London Underground is the largest of these, see Fourth Rail.

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 an 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 of electricity than steel) 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.)

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Advantages of third rail

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.

Third-rail systems cause less visual intrusion: they do not need overhead lines, which some people perceive as unsightly. Singapore, for example, has banned overhead lines outside tunnels.

While sometimes used in new transit system construction, third rails are now considerably less popular than overhead systems. In the U.S., they are still the usual means of powering heavy rapid transit lines that are completely grade-separated. Monorail typically use a variation of the third rail (with a fourth rail as well) for current transmission in the form of cables or other electrical conductors placed on the sides of the guideway and contacted by a sprung shoe.

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.

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Disadvantages of third rail

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

Safety: An unguarded electrified rail is a 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 urban legends that people have died while urinating on the third rail (the urine stream supposedly completes an electrical circuit that electrocutes the victim), although this was debunked by MythBusters. [2] However, such electrocution incidents have occured, including one that resulted in a lawsuit by the wife of Sang Yeul Lee, who urinated on a Chicago Transit Authority line while intoxicated and was electrocuted.

A new tramway system in Bordeaux surmounts the safety problem by using a third rail divided into insulated segments. Each segment is live only while completely covered by a tram, so there is no risk of a person or animal coming into contact with a live rail, see Third-rail power for trams.

Limited capacity: A relatively low voltage is necessary in a third-rail system — otherwise, electricity would arc from the rail to the ground or the running rails — but this low voltage means that electrical feeder sub-stations have to be set up at frequent intervals along the line, increasing operating costs. The low voltage also means that the system is prone to overload, which makes such systems unsuitable for freight or high-speed trains demanding high amounts of power. These 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 suburban railway route miles across south and southeast 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.

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. On the Chicago Transit Authority system, the jumper cables are known as stingers; they are insulated poles with a wired contact that may be manually pressed against contact shoes to restart a gapped train.

Additional infrastructure restrictions: On the Metro-North Railroad and the Long Island Rail Road (including Pennsylvania Station which is owned by Amtrak) the safety cover decreases the structure gauge and in turn the loading gauge.

Inefficient contact: Fallen leaves, snow and other debris on the conductor rail can reduce the efficiency of the contact between the conductor rail and the pickup shoes, leaving trains stalled because of the lack of power. However the bottom contact third rail, as used on the Metro-North Railroad (see also first paragraph of "Technical aspects" above), is all but immune to this problem.

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Compromise systems

There are and have been several systems in which third rail has been used for part of the system, and overhead lines for the remainder. These exist sometimes because of the connection of separately-owned railways using the different systems, or because of local ordinances.

In New York City, electric trains that must use third rail leaving Grand Central Terminal on the former New York Central Railroad (now Metro-North Railroad) switch to or from overhead lines when they need to operate out onto the former New York, New Haven and Hartford Railroad (now Amtrak) line to Connecticut. The switch is made "on the fly" controlled from the engineer's position.

The Blue Line of Boston's MBTA uses third rail electrification from the start of the line downtown to Airport, where it switches to overhead catenary for the remainder of the line to Wonderland.

The older lines in the west of the Oslo T-bane system were built with overhead lines (some since converted to third rail) while the eastern lines were built with third rail. Trains operating on the older lines can operate both with third rail and overhead lines.

Several types of British Railway trains operate on both overhead and third rail systems, including the class 313, 319, 325 and 373 Eurostar trains.

In Manhattan, New York City, and in Washington, D.C., local ordinances required electrified street railways to draw current from a third rail and return the current to a fourth rail, both installed in a continuous vault underneath the street and accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead. Some sections of the former London tram system also used the conduit current collection system, and here too there were some tramcars which could collect power from both overhead and under-road sources.

The newly built Tramway in the City of Bordeaux (France) uses a novel system with a third rail in the center of the Track. The third rail is separated into 8m (26ft-3in) long conducting and 3m (9ft-10in) long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the tram (activated by a coded signal sent by the train) and switch it off before it becomes exposed again. This system (called "Alimentation par Sol" (APS), meaning "current supply via ground") is used in the historic center of Bordeaux: suburban line routes use a conventional overhead lines, see also ground-level power supply.

In Chicago, the Yellow Line, also known as the Skokie Swift, operated for most of its distance with third rail, switching to overhead catenary before reaching the end of the line at the Dempster Street station. In 2004, the catenary portion was converted to third rail. This particular line was once a part of the Chicago, North Shore and Milwaukee interurban line.

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