wagon vs police wagon

wagon and police wagon both are nouns.

Word	Noun	Adjective	Verb	Adverb
wagon	Yes	No	No	No
police wagon	Yes	No	No	No

As nouns, wagon and police wagon are synonyms defined as:

Other synonyms of wagon include black Maria, paddy wagon, patrol wagon, police van.

wagon (noun)	police wagon (noun)
any of various kinds of wheeled vehicles drawn by an animal or a tractor	van used by police to transport prisoners
a car that has a long body and rear door with space behind rear seat
van used by police to transport prisoners
a child's four-wheeled toy cart sometimes used for coasting