freight car vs flat

freight car and flat both are nouns.

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

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

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

freight car (noun)	flat (noun)
a railway car that carries freight	a suite of rooms usually on one floor of an apartment house
	scenery consisting of a wooden frame covered with painted canvas; part of a stage setting
	a deflated pneumatic tire
	freight car without permanent sides or roof
	a shallow box in which seedlings are started
	a musical notation indicating one half step lower than the note named
	a level tract of land

freight car (adjective)	flat (adjective)
	(of a musical note) lowered in pitch by one chromatic semitone
	lacking contrast or shading between tones
	not reflecting light; not glossy
	having a surface without slope, tilt in which no part is higher or lower than another
	horizontally level
	commercially inactive
	lacking the expected range or depth; not designed to give an illusion or depth
	having lost effervescence
	having a relatively broad surface in relation to depth or thickness
	lacking taste or flavor or tang
	flattened laterally along the whole length (e.g., certain leafstalks or flatfishes)
	stretched out and lying at full length along the ground
	sounded or spoken in a tone unvarying in pitch
	not modified or restricted by reservations
	lacking stimulating characteristics; uninteresting

freight car (adverb)	flat (adverb)
	in a forthright manner; candidly or frankly
	with flat sails