|1.||an extra rail from which an electric train picks up current by means of a sliding collector to feed power to its motors|
|a. a cause or topic that is considered extremely dangerous for a person to support or comment on|
|b. (as modifier): a third-rail issue|
|a scrap or morsel of food left at a meal.|
|an arrangement of five objects, as trees, in a square or rectangle, one at each corner and one in the middle.|
Something that is dangerous to tamper with, as in Anything concerning veterans is a political third rail. This term alludes to the rail that supplies the high voltage powering an electric train, so called since 1918. On the other hand, grab hold of the third rail means "become energized." Both shifts from the original meaning date from the late 1900s.