underpass vs subway

underpass and subway both are nouns.

Word	Noun	Adjective	Verb	Adverb
underpass	Yes	No	No	No
subway	Yes	No	No	No

As nouns, underpass and subway are synonyms defined as:

underpass and subway: an underground tunnel or passage enabling pedestrians to cross a road or railway

underpass (noun)	subway (noun)
an underground tunnel or passage enabling pedestrians to cross a road or railway	an electric railway operating below the surface of the ground (usually in a city)
	an underground tunnel or passage enabling pedestrians to cross a road or railway