third-rail

listen to the pronunciation of third-rail
Английский Язык - Английский Язык
The electrified rail that runs besides, or between train tracks to power electric trains
anything that is dangerous to come into contact with, or is best avoided

Social security is the third rail of American politics: touch it and your political career dies.

a rail through which electric current is supplied to an electric locomotive
An electric railway using such a rail
The third rail used in the third-rail system