subway system vs subway

subway system and subway both are nouns.

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

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

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

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

subway system (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