subway system vs metro

subway system and metro both are nouns.

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

As nouns, subway system and metro are synonyms defined as:

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

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

subway system (noun)	metro (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)