rails vs third rail

rails and third rail both are nouns.

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

As nouns, third rail is a hyponym of rails; that is, third rail is a word with a more specific, narrower meaning than rails:

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

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