English Dictionary

fourth rail

noun

(rail 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.

The fourth rail is used for power supply in some railway systems.

意味（1）

(rail 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.

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!)