third rail vs rails

third rail and rails both are nouns.

Word	Noun	Adjective	Verb	Adverb
third rail	Yes	No	No	No
rails	Yes	No	No	No

As nouns, rails is a hypernym of third rail; that is, rails is a word with a broader meaning than third rail:

third rail: a rail through which electric current is supplied to an electric locomotive
rails: a bar or pair of parallel bars of rolled steel making the railway along which railroad cars or other vehicles can roll

Other hypernyms of third rail include rail, runway, track.

third rail (noun)	rails (noun)
a rail through which electric current is supplied to an electric locomotive	a bar or pair of parallel bars of rolled steel making the railway along which railroad cars or other vehicles can roll