- Railroads.
- 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.
- an additional running rail laid on the same ties as the two regular rails of a railroad track to provide a multigauge capability.
- a controversial topic or issue in the public sphere that people, especially politicians, try to avoid discussing (often used attributively): a third-rail subject;Doping is the third rail of the Olympics.Social Security is the third rail of American politics.
- an extra rail from which an electric train picks up current by means of a sliding collector to feed power to its motors
- US politics
- a cause or topic that is considered extremely dangerous for a person to support or comment on
- (as modifier)
