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1. The rail that supplies the high voltage to power a train on an electric railway.
2. A subject that tends to be avoided because of its offensive or controversial nature: "Social Security, the 'third rail' of American politics" (Charles Stein).
1. (Railways) an extra rail from which an electric train picks up current by means of a sliding collector to feed power to its motors
a. a cause or topic that is considered extremely dangerous for a person to support or comment on
b. (as modifier): a third-rail issue.
a rail in an electrified railroad that provides current to a car or locomotive.