Meaning of third rail

third' rail'

Pronunciation: [key]
— Railroads. Railroads.
  1. a rail laid parallel and adjacent to the running rails of an electrified railroad to provide electric current to the motors of a car or locomotive through contact shoes.
  2. an additional running rail laid on the same ties as the two regular rails of a railroad track to provide a multigauge capability.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.