hang back vs trail

hang back is not a noun while trail is a noun.

Word	Noun	Adjective	Verb	Adverb
hang back	No	No	Yes	No
trail	Yes	No	Yes	No

As verbs, hang back and trail are synonyms defined as:

Other synonyms of hang back include drag, drop back, drop behind, get behind.

hang back (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

hang back (verb)	trail (verb)
to lag or linger behind	drag loosely along a surface; allow to sweep the ground
	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