Or the four rails on the Underground
Just what you need when you build your railways, at the least the deep level ones', to travel through iron tubes. Adopted ultimately as the universal power transfer system, for even the "cut and cover" sub surface lines. There are stretches of the latter that shares their use with the more conventional 3 rail mainline stock. Max
From Wikipedia
The lines are electrified with a four-rail DC system. The configuration and potential of the conductor rails varies across the network. As of 2020, there are three different conductor rail configurations:
The original configuration is nominally 630 V with a −210 V centre conductor rail and a +420 V outside conductor rail. This is the default configuration wherever deep-level tube trains operate.[8]
Beginning in 2016, sections of the sub-surface network were reconfigured to a 750 V configuration (−250 V and +500 V rails). As of 2019, the entire sub-surface network uses this configuration except for Uxbridge to Finchley Road (via Harrow) as 1973 Stock and 1996 Stock (of the Piccadilly and Jubilee lines respectively) share the railway.[9]
In areas where Underground and third rail rolling stock share tracks, the centre conductor rail is electrically connected to the running rails. This still results in a 750 V voltage, but in a 0 V/+750 V configuration.[8] Lines configured in this manner include:
Between Gunnersbury and Richmond on the District line, shared with London Overground
Between East Putney and Wimbledon on a section of the District line that was transferred from British Rail
Between Queen's Park and Harrow & Wealdstone on the Watford DC line, served by Bakerloo line and London Overground trains
The four rail system was first used in the early 20th 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.[10]
The traction current has no direct earth point, but there are two resistors connected in series across the traction supply. The positive resistor is twice as great as the negative resistor, since the positive rail carries twice the voltage of the negative rail. The junction point of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are great enough to prevent large currents flowing through the earthed infrastructure.
