Green’s Dictionary of Slang

third-rail adj.

[see third rail n.1 ]

(US) dangerous.

[US]Chicago Trib. 19 July in Fleming Unforgettable Season (1981) 128: Hero Tinker is a regular little third-rail athlete. His home run Friday led to the destruction of one fan, and a two-bagger yesterday [...] caused an overwrought bug to throw a fit.