railroad car vs railcar

railroad car and railcar both are nouns.

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

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

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

Other synonyms of railroad car include car, railway car.

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