metro vs subway system

metro and subway system both are nouns.

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

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

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

Other synonyms of metro include subway, tube, underground.

metro (noun)	subway system (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)