monorail vs railroad line

monorail and railroad line both are nouns.

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

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

monorail: a railway having a single track
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 monorail include railroad, railway, railway line, railway system.

monorail (noun)	railroad line (noun)
a railway having a single track	line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight