car vs mail car

car and mail car both are nouns.

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

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

car (noun)	mail car (noun)
a motor vehicle with four wheels; usually propelled by an internal combustion engine	a railway car in which mail is transported and sorted
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