dining car vs diner

dining car and diner both are nouns.

Word	Noun	Adjective	Verb	Adverb
dining car	Yes	No	No	No
diner	Yes	No	No	No

As nouns, dining car and diner are synonyms defined as:

Other synonyms of dining car include buffet car, dining compartment.

dining car (noun)	diner (noun)
a passenger car where food is served in transit	a passenger car where food is served in transit
	a restaurant that resembles a dining car
	a person eating a meal (especially in a restaurant)