double-deck railcar vs double-deck rail car

double-deck railcar and double-deck rail car both are nouns.

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

As nouns, double-deck railcar and double-deck rail car are synonyms defined as:

double-deck railcar and double-deck rail car: a type of rail car that has two levels of passenger accommodation, as opposed to one, increasing passenger capacity.

Other synonyms of double-deck railcar include bilevel car, bilevel rail car, bilevel railcar, double-deck carriage.

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