bilevel railcar vs railroad car

bilevel railcar and railroad car both are nouns.

Word	Noun	Adjective	Verb	Adverb
bilevel railcar	Yes	No	No	No
railroad car	Yes	No	No	No

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

bilevel railcar: a type of rail car that has two levels of passenger accommodation, as opposed to one, increasing passenger capacity.
railroad car: a wheeled vehicle adapted to the rails of railroad

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

bilevel railcar (noun)	railroad car (noun)
a type of rail car that has two levels of passenger accommodation, as opposed to one, increasing passenger capacity.	a wheeled vehicle adapted to the rails of railroad