railroad tunnel vs tunnel

railroad tunnel and tunnel both are nouns.

Word	Noun	Adjective	Verb	Adverb
railroad tunnel	Yes	No	No	No
tunnel	Yes	No	Yes	No

As nouns, tunnel is a hypernym of railroad tunnel; that is, tunnel is a word with a broader meaning than railroad tunnel:

railroad tunnel: a tunnel through which the railroad track runs
tunnel: a passageway through or under something, usually underground (especially one for trains or cars)

railroad tunnel (noun)	tunnel (noun)
a tunnel through which the railroad track runs	a passageway through or under something, usually underground (especially one for trains or cars)
	a hole made by an animal, usually for shelter

railroad tunnel (verb)	tunnel (verb)
	force a way through
	move through by or as by digging