mail car vs railcar

mail car and railcar both are nouns.

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

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

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

mail car (noun)	railcar (noun)
a railway car in which mail is transported and sorted	a wheeled vehicle adapted to the rails of railroad