rack railway vs railroad line

rack railway and railroad line both are nouns.

Word	Noun	Adjective	Verb	Adverb
rack railway	Yes	No	No	No
railroad line	Yes	No	No	No

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

rack railway: railway for steep mountains; a cogwheel on the locomotive engages cogs on a center rail to provide traction
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 rack railway include railroad, railway, railway line, railway system.

rack railway (noun)	railroad line (noun)
railway for steep mountains; a cogwheel on the locomotive engages cogs on a center rail to provide traction	line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight