cog railway vs railroad line

cog railway and railroad line both are nouns.

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

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

cog 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 cog railway include railroad, railway, railway line, railway system.

cog 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