1. The rail that supplies the high voltage to power a train on an electric railway.
2. A subject that tends to be avoided because of its offensive or controversial nature: "Social Security, the 'third rail' of American politics" (Charles Stein).

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.