A third rail is the rail of a three-rail electric railway line separated to the two rails (the running rails) which carry trains, that carries the current to power the trains. Electric trains on such a system have a "shoe" that contacts the third rail to collect the current, which is returned in one of the two running rails. Some railways have a fourth rail for the return of the current.
Voltage in the third rail is typically 600-1500 volts DC (direct current), so touching it while grounded usually proves fatal.
The "third rail" of politics is an issue so potent and one-sided that no politician dares challenge the traditional view. Social Security and other benefits for senior citizens, who vote in greater proportion to other age groups, represent the classic third rail of politics.
The term derives from the railway use.