cars would be returned to their point of origin empty. If not in demand, the cars would be cleaned and possibly used for a dry shipment. Refrigerator cars required effective insulation to protect their contents from temperature extremes. "Hairfelt" derived from compressed cattle hair, sandwiched into the floor and walls of the car, was inexpensive, yet flawed over its three- to four-year service life it would decay, rotting out the car's wooden partitions and tainting the cargo with a foul odor. The higher cost of other materials such as "Linofelt" (woven from flax fibers) or cork prevented their widespread adoption. Synthetic materials such as fiberglass and polystyrene foam, both introduced after World War II, offered the most cost-effective and practical solution. The United States Office of Defense Transportation implemented mandatory pooling of class RS produce refrigerator cars from 1941 through 1948. World War II experience found the cars spending 60 percent of their time traveling loaded, 30 percent traveling empty, and 10 percent idle; and indicated the average 14 loads each car carried per year included 5 requiring bunker icing, 1 requiring heating, and 8 using ventilation or top icing. Following experience with assorted car specifications, the United Fresh Fruit and Vegetable Association (UFF&VA) listed what they considered the best features of ice refrigerator cars in 1948: Steel cars (vs wood) for better insulation protection and greater rigidity resulting in reduced leakage around doors A minimum of insulation thickness with all insulation protected from moisture Cushioned trucks and draft gear to minimize jarring and bruising of produce Standardized interior dimensions to allow improved loading methods with standardized containers Adjustable ice bunker bulkheads to allow greater floor space for shippers using top icing alone Vertically adjustable grates within the ice bunkers to allow half-stage icing to reduce icing charges where appropriate Forced air circulation within the car An additional lining to allow side wall flues circulating air around all cargo preventing contact with exterior car walls Perforated floor racks providing similar protection and air circulation under the cargo Provisions for pre-cooling the cars with a portable unit at the loading platforms. Mechanical refrigeration In the latter half of the 20th century, mechanical refrigeration began to replace ice-based systems. Soon after, mechanical refrigeration units replaced the armies of personnel required to re-ice the cars. The sliding plug door was introduced experimentally by P.F.E. (Pacific Fruit Express) in April 1947, when one of