reduction gear vs geartrain

reduction gear and geartrain both are nouns.

Word	Noun	Adjective	Verb	Adverb
reduction gear	Yes	No	No	No
geartrain	Yes	No	No	No

As nouns, geartrain is a hypernym of reduction gear; that is, geartrain is a word with a broader meaning than reduction gear:

reduction gear: gearing that reduces an input speed to a slower output speed
geartrain: wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed

Other hypernyms of reduction gear include gear, gearing, power train, train.

reduction gear (noun)	geartrain (noun)
gearing that reduces an input speed to a slower output speed	wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed