subway vs subway system

subway and subway system both are nouns.

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

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

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

Other synonyms of subway include metro, tube, underground.

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