mail car vs car

mail car and car both are nouns.

Word	Noun	Adjective	Verb	Adverb
mail car	Yes	No	No	No
car	Yes	No	No	No

As nouns, car is a hypernym of mail car; that is, car is a word with a broader meaning than mail car:

Other hypernyms of mail car include railcar, railroad car, railway car.

mail car (noun)	car (noun)
a railway car in which mail is transported and sorted	a motor vehicle with four wheels; usually propelled by an internal combustion engine
	a wheeled vehicle adapted to the rails of railroad
	where passengers ride up and down
	the compartment that is suspended from an airship and that carries personnel and the cargo and the power plant
	a conveyance for passengers or freight on a cable railway