- 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”, 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.
the electrified rail that runs beside or between train tracks to power electric trains