freight car vs coal car

freight car and coal car both are nouns.

Word	Noun	Adjective	Verb	Adverb
freight car	Yes	No	No	No
coal car	Yes	No	No	No

As nouns, coal car is a hyponym of freight car; that is, coal car is a word with a more specific, narrower meaning than freight car:

Other hyponyms of freight car include boxcar, cattle car, flat, flatbed, flatcar, gondola, gondola car, refrigerator car, tank, tank car.

freight car (noun)	coal car (noun)
a railway car that carries freight	freight car with fixed sides and no roof; for transporting coal