third rail

noun2

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
2. •
   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