third rail

third rail

noun Railroads.
1.
a rail laid parallel and adjacent to the running rails of an electrified railroad to provide electric current to the motors of a car or locomotive through contact shoes.
2.
an additional running rail laid on the same ties as the two regular rails of a railroad track to provide a multigauge capability.

Origin:
1865–70, Americanism

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
third rail
 
n
1.  an extra rail from which an electric train picks up current by means of a sliding collector to feed power to its motors
2.  (US) politics
 a.  a cause or topic that is considered extremely dangerous for a person to support or comment on
 b.  (as modifier): a third-rail issue

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

third rail

Something that is dangerous to tamper with, as in Anything concerning veterans is a political third rail. This term alludes to the rail that supplies the high voltage powering an electric train, so called since 1918. On the other hand, grab hold of the third rail means "become energized." Both shifts from the original meaning date from the late 1900s.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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