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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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
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