subway vs underpass

subway and underpass both are nouns.

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

As nouns, subway and underpass are synonyms defined as:

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

subway (noun)	underpass (noun)
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
an underground tunnel or passage enabling pedestrians to cross a road or railway