fourth rail

noun

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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Meanings

nounfourth rail (plural fourth rails)

plural fourth rails

  1. rail transportAn 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.

    The railway used the 'Third Rail' system of electrification, like all the early Tubes. (But it would soon be replaced across the network by another system even more exciting - Fourth Rail!)

    2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 103
    1 more example

    It was the only line to operate by means of the third-rail system (as distinct from the fourth-rail system now universal on all London's Underground railways) and its coaches were of different design and appearance from those on other systems.

    1948 January and February, C. R. L. Coles, “The Grouping Era”, in Railway Magazine, page 21
Cite this entry
APA

fourth rail. (n.d.). In WatkinsX Dictionary. Retrieved November 3, 2026, from https://dictionary.watkinsx.com/word/fourth%20rail

MLA

“fourth rail.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/fourth%20rail. Accessed November 3, 2026.

Chicago

WatkinsX Dictionary, s.v. “fourth rail,” accessed November 3, 2026, https://dictionary.watkinsx.com/word/fourth%20rail.

Source: fourth rail on Wiktionary, CC BY-SA 4.0.