fourth rail
noun1
Rail transport, Railways, Transport 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.
- 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:
- 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: