train vs retrain

train is a noun but retrain is not a noun.

Word	Noun	Adjective	Verb	Adverb
train	Yes	No	Yes	No
retrain	No	No	Yes	No

As verbs, retrain is a hyponym of train; that is, retrain is a word with a more specific, narrower meaning than train:

train: undergo training or instruction in preparation for a particular role, function, or profession
retrain: train again

Other hyponyms of train include apprentice, drill.

As verbs, retrain is a hyponym of train; that is, retrain is a word with a more specific, narrower meaning than train:

Other hyponyms of train include drill, house-train, housebreak, toilet-train.

train (noun)	retrain (noun)
public transport provided by a line of railway cars coupled together and drawn by a locomotive
wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed
piece of cloth forming the long back section of a gown that is drawn along the floor
a series of consequences wrought by an event
a procession (of wagons or mules or camels) traveling together in single file
a sequentially ordered set of things or events or ideas in which each successive member is related to the preceding

train (verb)	retrain (verb)
exercise in order to prepare for an event or competition	train again
undergo training or instruction in preparation for a particular role, function, or profession	teach new skills
create by training and teaching
teach and supervise (someone); act as a trainer or coach (to), as in sports
point or cause to go (blows, weapons, or objects such as photographic equipment) towards
drag loosely along a surface; allow to sweep the ground
travel by rail or train
educate for a future role or function
teach or refine to be discriminative in taste or judgment
develop (a child's or animal's) behavior by instruction and practice; especially to teach self-control
cause to grow in a certain way by tying and pruning it