narrow gauge vs railroad track

narrow gauge and railroad track both are nouns.

Word	Noun	Adjective	Verb	Adverb
narrow gauge	Yes	No	No	No
railroad track	Yes	No	No	No

As nouns, railroad track is a hypernym of narrow gauge; that is, railroad track is a word with a broader meaning than narrow gauge:

narrow gauge: a railroad track (or its width) narrower than the standard 56.5 inches
railroad track: a line of track providing a runway for wheels

Other hypernyms of narrow gauge include railroad, railway, gauge.

narrow gauge (noun)	railroad track (noun)
a railroad track (or its width) narrower than the standard 56.5 inches	a line of track providing a runway for wheels