rail line vs spur

rail line and spur both are nouns.

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

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

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

rail line (noun)	spur (noun)
the road consisting of railroad track and roadbed	a sharp prod fixed to a rider's heel and used to urge a horse onward
	a railway line connected to a trunk line
	a verbalization that encourages you to attempt something
	tubular extension at the base of the corolla in some flowers
	any sharply pointed projection