rack railway vs railway line

rack railway and railway line both are nouns.

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

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

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