> The term "third rail" comes from underground subway systems where there is a third, electrified rail to provide power to trains. There are usually warnings in

> The term "third rail" comes from underground subway systems where there is a third, electrified rail to provide power to trains. There are usually warnings in the subway not to touch the rail due to the risk of electrocution. "The third rail" is used colloquially to describe a topic that is socially or politically difficult to discuss because of the reactions it can bring up. An older metaphor for the same thing: minefield.