double-deck rail car vs railcar

double-deck rail car and railcar both are nouns.

Word	Noun	Adjective	Verb	Adverb
double-deck rail car	Yes	No	No	No
railcar	Yes	No	No	No

As nouns, railcar is a hypernym of double-deck rail car; that is, railcar is a word with a broader meaning than double-deck rail car:

double-deck rail car: a type of rail car that has two levels of passenger accommodation, as opposed to one, increasing passenger capacity.
railcar: a wheeled vehicle adapted to the rails of railroad

Other hypernyms of double-deck rail car include car, railroad car, railway car.

double-deck rail car (noun)	railcar (noun)
a type of rail car that has two levels of passenger accommodation, as opposed to one, increasing passenger capacity.	a wheeled vehicle adapted to the rails of railroad