third rail

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The rail that supplies the high voltage to power a train on an electric railway.
• noun A subject that tends to be avoided because of its offensive or controversial nature.

from the GNU version of the Collaborative International Dictionary of English.

• colloq., colloq. The third rail used in the third-rail system.
• colloq. An electric railway using such a rail.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The electrified rail that runs besides, or between train tracks to power electric trains.
• noun by extension anything that is dangerous to come into contact with, or is best avoided

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a rail through which electric current is supplied to an electric locomotive