railway car vs carriage

railway car and carriage both are nouns.

Word	Noun	Adjective	Verb	Adverb
railway car	Yes	No	No	No
carriage	Yes	No	No	No

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

railway car (noun)	carriage (noun)
a wheeled vehicle adapted to the rails of railroad	a vehicle with wheels drawn by one or more horses
	a railcar where passengers ride
	a machine part that carries something else
	a small vehicle with four wheels in which a baby or child is pushed around
	characteristic way of bearing one's body