freight car vs tank car

freight car and tank car both are nouns.

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

As nouns, tank car is a hyponym of freight car; that is, tank car 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, flatcar, gondola, gondola car, refrigerator car, tank.

freight car (noun)	tank car (noun)
a railway car that carries freight	a freight car that transports liquids or gases in bulk