Why does the tube have 4 rails?

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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Hampstead is the deepest station below the surface, at 58.5 metres (192 ft), as its surface building is near the top of a hill, and the Jubilee line platforms at Westminster are the deepest platforms below sea level at 32 metres (105 ft).

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The lines to the south were electrified earlier, when third rail was the better option. It also means there is no impact on bridges across the railway. The later electrification to the north of London used overhead power lines at a higher voltage.

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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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Revealed: These are London's most overcrowded Tube lines Statistics from mayor of London Sadiq Khan show that morning peak hour capacity on the London Underground is more than 100 per cent, with the Northern, Central and Jubilee lines the most crowded.

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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 Elizabeth line is unique on the London Underground in that surface stock trains – as big as the S stock than run on the Metropolitan and District lines – run in tube tunnels under the Capital, and far out into Berkshire and Essex on the surface.

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Andy Morris survives 625-volt zap after being pushed onto live third rail in drunken brawl.

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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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Most of the lines emerge on the surface outside the Central London area. The oldest trains currently in service on the Underground are 1972 Stock trains on the Bakerloo line. The Underground is electrified using a four-rail system, the DC traction supply being independent of the running rails.

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How to Survive a Fall Onto Subway Tracks
  1. Look for others who can help you back onto the platform by offering a hand and pulling you up.
  2. Tuck into an alcove or underneath the platform where you can fit.
  3. Stand between two sets of tracks where there is typically enough clearance to be safe.


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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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The number of route miles electrified in these years was answered to a written question in parliament. In November 2019 the annual statistics for route miles electrified was published by the DfT and shows that 38% of the UK network is now electrified.

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