- The electrified rail that runs beside or between train tracks to power electric trains.
- (by extension) anything that is dangerous to come into contact with, or is best avoided.
- Social security is the third rail of American politics: touch it and your political career dies.
2014 March 29, Stephen Sestanovich, “Putin's reckless gamble”, in The New York Times:
- Despite ethnic resentment and suspicion, they [Ukraine's leaders] never contemplated breaking up the country. This was the third rail of Ukranian politics, and very few were willing to touch it. Secession was taboo.