What is 4th rail?

fourth rail (plural fourth rails) (rail transport) An extra rail in addition to the third rail (live rail) which is used for current return purposes, mainly by London Underground, because of problems caused by using the running rails for current return underground.



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In 3rd rail systems, the current returns to the supply though the ordinary running rail(s). In a 4th rail system, a second insulated rail is provided, and current return is via a second set of pickup shoes through to the second insulated rail.

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The four rail system was first used in the early twentieth century. The isolated traction current return allowed a train's position to be detected using DC track circuits, and reduced any earth leakage currents that could affect service pipes, telephone cables, or cast iron tunnel liners.

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More than 30% of the UK rail network currently uses a third rail to power the train, commonly in the southeast and in dense urban areas around the country. One of the significant advantages of third rail electrification is its cost-effectiveness.

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A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track.

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Third rail systems are also less vulnerable to weather-related disruptions that affect overhead wires. As the third rail is on the ground it is unaffected by strong winds and less exposed to heavy snowfall.

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The deepest station is Hampstead on the Northern line, which runs down to 58.5 metres. 15. In Central London the deepest station below street level is also the Northern line. It is the DLR concourse at Bank, which is 41.4 metres below.

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The Metropolitan line is the oldest underground railway in the world. The Metropolitan Railway opened in January 1863 and was an immediate success, though its construction took nearly two years and caused huge disruption in the streets.

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The longest distance between stations is on the Metropolitan line from Chesham to Chalfont & Latimer: a total of only 3.89 miles. 11.

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All London Underground Lines (including the W & C) operate at 630 volts DC using third (positive) and fourth (negative) current rails. The current rails are positioned so that the contact surface is higher than the running rails.

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The Elizabeth line will use Class 345 trains, which are around 200 metres long–making them more than one and a half times longer than the current longest tube train.

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Implementing high-speed rail will keep billions of dollars in the U.S. economy by decreasing the amount of oil that the U.S. consumes. According to the International Association of Railways (UIC), high-speed rail is eight times more energy efficient than airplanes and four times more efficient than automobile use.

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Compared to other popular forms of travel, such as cars, ships, buses, and planes, trains are one of the safest forms of transportation in the United States.

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This is why birds do not die from landing on power lines, and subway rats do not get electrocuted even if they run across the third rail; they are not bridging the energized wire or rail to a grounded part of the structure.

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TIL Rats and birds don't get electrocuted by the third rail because they don't form a grounding connection. This is not some property inherent to rats and birds. This is because when they touch the third rail, they aren't touching anything else.

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