I don't think there were any cars built specifically for prisoners. In the US, POWs from Germany and Italy were taken to camps in regular service or extra passenger trains using whatever equipment was available. Many times American troops would get off the trains and onto ships, their seats taken for the trip to the interior by Germans on their way to farm work for the war's duration.
in Germany, the cars used for Hitler's "final solution" were just regular boxcars and cattle cars, same with the Soviet Union. Many of these were used in regular service after the war as well, and there are some preserved at some of the concentration camp memorials around Europe.
