choo-choo vs locomotive engine

choo-choo and locomotive engine both are nouns.

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

As nouns, locomotive engine is a hypernym of choo-choo; that is, locomotive engine is a word with a broader meaning than choo-choo:

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

Other hypernyms of choo-choo include engine, locomotive, railway locomotive.

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