fall behind vs trail

fall behind is not a noun while trail is a noun.

Word	Noun	Adjective	Verb	Adverb
fall behind	No	No	Yes	No
trail	Yes	No	Yes	No

As verbs, trail is a hyponym of fall behind; that is, trail is a word with a more specific, narrower meaning than fall behind:

fall behind: hang (back) or fall (behind) in movement, progress, development, etc.
trail: to lag or linger behind

Other hyponyms of fall behind include drag, drop back, drop behind, get behind, hang back.

fall behind (noun)	trail (noun)
	a path or track roughly blazed through wild or hilly country
	evidence pointing to a possible solution
	a track or mark left by something that has passed

fall behind (verb)	trail (verb)
retreat	drag loosely along a surface; allow to sweep the ground
hang (back) or fall (behind) in movement, progress, development, etc.	hang down so as to drag along the ground
	go after with the intent to catch
	move, proceed, or walk draggingly or slowly
	to lag or linger behind