freight car vs flatbed

freight car and flatbed both are nouns.

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

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

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

freight car (noun)	flatbed (noun)
a railway car that carries freight	an open truck bed or trailer with no sides; used to carry large heavy objects
	freight car without permanent sides or roof