engine vs choo-choo

engine and choo-choo both are nouns.

Word	Noun	Adjective	Verb	Adverb
engine	Yes	No	No	No
choo-choo	Yes	No	No	No

As nouns, choo-choo is a hyponym of engine; that is, choo-choo is a word with a more specific, narrower meaning than engine:

engine: a wheeled vehicle consisting of a self-propelled engine that is used to draw trains along railway tracks
choo-choo: a child's word for locomotive

engine (noun)	choo-choo (noun)
motor that converts thermal energy to mechanical work	a child's word for locomotive
a wheeled vehicle consisting of a self-propelled engine that is used to draw trains along railway tracks
an instrument or machine that is used in warfare, such as a battering ram, catapult, artillery piece, etc.
something used to achieve a purpose