railcar vs railroad car

railcar and railroad car both are nouns.

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

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

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

Other synonyms of railcar include car, railway car.

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