axle vs trailing axle

axle and trailing axle both are nouns.

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

As nouns, trailing axle is a hyponym of axle; that is, trailing axle is a word with a more specific, narrower meaning than axle:

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

Other hyponyms of axle include dead axle, journal, driving axle, live axle, blind axle.

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