police wagon vs black Maria

police wagon and black Maria both are nouns.

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

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

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

police wagon (noun)	black Maria (noun)
van used by police to transport prisoners	van used by police to transport prisoners