Sliding door box cars were basically the build standard prior to World War II. It wasn?t until after the war that many railroads experimented with the use of plug doors, similar to those used on many refrigerator cars, on their box cars. The tight seal of the plug door, accompanied by insulated sides, ends, floor and roof panels, allowed these new box cars to carry perishables where temperature control was important but refrigeration wasn?t necessary. This design, classified by the AAR (Association of American Railways) as ?RB? or ?RBL?, meaning bunkerless refrigerator cars, gained popularity in the mid 1950s and the following decade saw over 3,000 similar cars built for a number of different owners.