third rail

noun

1. Railroads.
   1. a rail laid parallel and adjacent to the running rails of an electrified railroad to provide electric current to the motors of a car or locomotive through contact shoes.
   2. an additional running rail laid on the same ties as the two regular rails of a railroad track to provide a multigauge capability.
2. a controversial topic or issue in the public sphere that people, especially politicians, try to avoid discussing (often used attributively): a third-rail subject;Doping is the third rail of the Olympics.Social Security is the third rail of American politics.

noun

1. an extra rail from which an electric train picks up current by means of a sliding collector to feed power to its motors
2. US politics
   1. a cause or topic that is considered extremely dangerous for a person to support or comment on
   2. (as modifier)a third-rail issue

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.