railcar vs railway car

railcar and railway car both are nouns.

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

As nouns, railcar and railway car are synonyms defined as:

railcar and railway car: a wheeled vehicle adapted to the rails of railroad

Other synonyms of railcar include car, railroad car.

railcar (noun)	railway car (noun)
a wheeled vehicle adapted to the rails of railroad	a wheeled vehicle adapted to the rails of railroad