The Collaborative International Dictionary of English v.0.48:

paddy wagon \paddy wagon\ n.
   An enclosed truck used by police to transport prisoners.

   Syn: patrol wagon, Black Maria.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

paddy wagon
    n 1: van used by police to transport prisoners [syn: police
         van, police wagon, paddy wagon, patrol wagon,
         wagon, black Maria]