freight car vs flatcar

freight car and flatcar both are nouns.

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

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

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

freight car (noun)	flatcar (noun)
a railway car that carries freight	freight car without permanent sides or roof