third rail

Noun

third rail (plural third rails)

1. The electrified rail that runs beside or between train tracks to power electric trains.
   • The system voltage is 630 volts d.c. and the supply to the trains is by top-contact third rail with running rail return.1956 October, “Recent Changes on the Tyneside Electrified Lines”, in Railway Magazine, page 659
   • The locomotive third-rail pick up shoes are of the new standard S.R. type. [...] Whenever the locomotive was working hard there was unmistakable evidence of its higher power than its predecessors in the brilliant and explosive arcing between conductor shoes and the third rail; this was particularly...1959 March, “The 2,500 h.p. electric locomotives for the Kent Coast electrification”, in Trains Illustrated, page 125

   Synonyms:

   conductor rail, live rail

2. (broadly) Anything that is controversial or dangerous to come into contact with, or is best avoided.
   • tax hikes as a third rail in U.S. politics
   • Today's rapidly developing third rail (at least in some states) is supporting driver's licenses for illegal immigrants, as favored by New York Gov. Eliot Spitzer.2007 November 12, Jonathan Alter, “Congenital Lawyer Redux”, in Newsweek, page 48