From The Collaborative International Dictionary of English v.0.48 :
patrol wagon \patrol wagon\ n.
An enclosed truck used by police to transport prisoners.
Syn: paddy wagon, Black Maria.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
patrol wagon
n 1: van used by police to transport prisoners [syn: police
van, police wagon, paddy wagon, patrol wagon,
wagon, black Maria]