metro vs railroad line

metro and railroad line both are nouns.

Word	Noun	Adjective	Verb	Adverb
metro	Yes	No	No	No
railroad line	Yes	No	No	No

As nouns, railroad line is a hypernym of metro; that is, railroad line is a word with a broader meaning than metro:

metro: 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 metro include railroad, railway, railway line, railway system.

metro (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