black Maria vs police wagon

black Maria and police wagon both are nouns.

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

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

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

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