rail line vs spur track

rail line and spur track both are nouns.

Word	Noun	Adjective	Verb	Adverb
rail line	Yes	No	No	No
spur track	Yes	No	No	No

As nouns, spur track is a hyponym of rail line; that is, spur track is a word with a more specific, narrower meaning than rail line:

Other hyponyms of rail line include branch line, spur, trunk line, trunk route.

rail line (noun)	spur track (noun)
the road consisting of railroad track and roadbed	a railway line connected to a trunk line