subway vs railway system

subway and railway system both are nouns.

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

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

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

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

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