rail line vs trunk line

rail line and trunk line both are nouns.

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

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

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

rail line (noun)	trunk line (noun)
the road consisting of railroad track and roadbed	a telephone line connecting two exchanges directly
	line that is the main route on a railway