spur track vs loop-line

spur track and loop-line both are nouns.

Word	Noun	Adjective	Verb	Adverb
spur track	Yes	No	No	No
loop-line	Yes	No	No	No

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

spur track: a railway line connected to a trunk line
loop-line: a railway branch line that branches from the trunk line and then rejoins it later on

spur track (noun)	loop-line (noun)
a railway line connected to a trunk line	a railway branch line that branches from the trunk line and then rejoins it later on