car vs railroad car

car and railroad car both are nouns.

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

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

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

Other synonyms of car include railcar, railway car.

car (noun)	railroad car (noun)
a motor vehicle with four wheels; usually propelled by an internal combustion engine	a wheeled vehicle adapted to the rails of railroad
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