patrol wagon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An enclosed police truck used to convey prisoners.

from The Century Dictionary.

• noun A large, covered, two-horse wagon belonging to the police force, used to carry prisoners to and from jail and for various other purposes.
• noun A long, uncovered, two-horse wagon used by the fire-insurance patrol to carry men, with tarpaulins, etc., to fires, for the protection of insured goods.

from the GNU version of the Collaborative International Dictionary of English.

• noun An enclosed truck used by police to transport prisoners.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun van used by police to transport prisoners