metro vs subway

metro and subway both are nouns.

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

As nouns, metro and subway are synonyms defined as:

metro and subway: an electric railway operating below the surface of the ground (usually in a city)

Other synonyms of metro include subway system, tube, underground.

metro (noun)	subway (noun)
an electric railway operating below the surface of the ground (usually in a city)	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