subway system vs railway line

subway system and railway line both are nouns.

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

As nouns, railway line is a hypernym of subway system; that is, railway line is a word with a broader meaning than subway system:

subway system: an electric railway operating below the surface of the ground (usually in a city)
railway line: line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight

Other hypernyms of subway system include railroad, railroad line, railway, railway system.

subway system (noun)	railway line (noun)
an electric railway operating below the surface of the ground (usually in a city)	the road consisting of railroad track and roadbed
	line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight