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**OUTDATED** London Underground Track (4th Rail)

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4th Rail track as seen on the London Underground. The 4th rail (center) is used as a dedicated return current rail which is isolated from the ground within tunnels. This is to avoid voltage leaking into the metal tunnels which can react and weaken the tunnels, which is obviously not good.

Different parts of the underground use different voltages over both the 3rd and 4th rail to power trains, depending on the Underground stock used and whether mainline trains have to use the same track.

Most track is electrified at 630V, with +420V on the outside rail and -210V on the center rail, but there are a few exceptions:
Most sections of the sub-surface lines (Metropolitan, Circle, District and Hammersmith & City) now runs at 750V with the new S-Stock trains, running at +500V on the outside and -250V on the center rail. The only section still running at the lower voltage is the section "shared" with the Jubilee line and shared with the Piccadilly line, from Finchley Road to Uxbridge, as the older trains cannot run at 750V.
The District line from Richmond to Gunnersbury & Wimbledon to East Putney, and the Bakerloo line from Queen's Park to Harrow & Wealdstone all run at 750V, but instead at +750V on the outside and -0V on the center rail. The center rail is also electrically connected to the running rails, meaning both 4th and 3rd rail trains send their voltage down the center and running rails, while not needing a shoe for the 4th rail on mainline trains or having to send voltage through the wheels on a tube train. While Wimbledon to East Putney is only officially served by the District Line, there is a daily parliamentary South Western Railway service to Waterloo which is why the track still runs at 750V.

Ground Track:
Wooden sleepers, 120km/h for sections shared with London Overground.
Viaduct Track:
Concrete sleepers, 120km/h for sections shared with London Overground.
Tunnel Track:
(Short?) sleepers, 50km/h, and isolation pots to avoid current leakage.

All track is cheaper than the medium track counterparts, due to the lower speeds, and tube tunnels being alot smaller and slower than the mainline counterparts.

Enjoy!
~Skye[skyehaynes.me]
View my other NIMBY Rails content[skyehaynes.me]