subway system vs railroad line

subway system and railroad line both are nouns.

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

As nouns, railroad line is a hypernym of subway system; that is, railroad 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)
railroad 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, railway, railway line, railway system.

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