drop back vs trail

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

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

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

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

drop 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

drop back (verb)	trail (verb)
take position in the rear, as in a military formation or in the line of scrimmage in football	drag loosely along a surface; allow to sweep the ground
to lag or linger behind	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