railroad car vs car

railroad car and car both are nouns.

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

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

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

Other synonyms of railroad car include railcar, railway car.

railroad car (noun)	car (noun)
a wheeled vehicle adapted to the rails of railroad	a motor vehicle with four wheels; usually propelled by an internal combustion engine
	a wheeled vehicle adapted to the rails of railroad
	where passengers ride up and down
	the compartment that is suspended from an airship and that carries personnel and the cargo and the power plant
	a conveyance for passengers or freight on a cable railway