black Maria vs police van

black Maria and police van both are nouns.

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

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

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

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