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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
1865-1870
1865-70, Americanism
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for third rail

third rail

noun
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)
  1. a cause or topic that is considered extremely dangerous for a person to support or comment on
  2. (as modifier): a third-rail issue
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Idioms and Phrases with third rail

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® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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