Meaning of fourth rail | Babel Free

Definitions

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.

Examples

“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.”

“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!)”

CEFR level