Definition of the word third rail

The term "third rail" originally referred to an electrical conductor in a train system that is located between the two traditional "rails" (or power rails) that electrified trains use to operate. Unlike the other rails, which are only connected to one of the power sources at a time, the third rail is always energized with high voltage electricity. Due to the high voltage and potential danger it poses, the third rail was dubbed the "third rail" because it is avoided by the train's wheels, much as a person would avoid touching a third, live wire in an electrical circuit. The term has since been used metaphorically to describe something that is risky, dangerous, or off-limits, such as a taboo topic or a sensitive political issue.