fourth rail (plural fourth rails) (rail transport) An extra rail in addition to the third rail

third rail

The third rail of a nation's politics is a metaphor for any issue so controversial that it is "charged" and "untouchable" to the extent that any politician or public official who dares to broach the subject will invariably suffer politically.

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(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. quotations ▼

How does 4th rail work?

With top-contact third (and fourth) rail a heavy shoe attached to the underside of a wooden beam which in turn is attached to the bogie, collects power by sliding over the top surface of the conductor rail.

Why does the tube have 4 rails?

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.