third rail

1. N-COUNT The third rail on a railway track is the steel bar which passes electric current to a train. (鉄路軌道)第三軌; 輸電軌

2. N-SING If you refer to something as the third rail of a political situation, you mean that it would be dangerous to make changes to that thing because people feel very strongly about it. (政治上的)險侷[美國英語] ['the' N]

Politicians speak of Social Security as the third rail. To touch it is to commit political suicide. 社保對政客們來說是個高危帶電的話題，碰它就等於政治自殺。