trailing axle vs axle

trailing axle and axle both are nouns.

Word	Noun	Adjective	Verb	Adverb
trailing axle	Yes	No	No	No
axle	Yes	No	No	No

As nouns, axle is a hypernym of trailing axle; that is, axle is a word with a broader meaning than trailing axle:

trailing axle: on a steam locomotive, an axle connecting a coupled pair of unpowered wheels located behind the driving wheels.
axle: a shaft on which a wheel rotates

trailing axle (noun)	axle (noun)
on a steam locomotive, an axle connecting a coupled pair of unpowered wheels located behind the driving wheels.	a shaft on which a wheel rotates