passenger car vs railroad car

passenger car and railroad car both are nouns.

Word	Noun	Adjective	Verb	Adverb
passenger car	Yes	No	No	No
railroad car	Yes	No	No	No

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

Other hypernyms of passenger car include car, railcar, railway car.

passenger car (noun)	railroad car (noun)
a railcar where passengers ride	a wheeled vehicle adapted to the rails of railroad