their R-40-10 series cars, #42626, was equipped with one. P.F.E.'s R-40-26 series reefers, designed in 1949 and built in 1951, were the first production series cars to be so equipped. In addition, the Santa Fe Railroad first used plug doors on their SFRD RR-47 series cars, which were also built in 1951. This type of door provided a larger six foot opening to facilitate car loading and unloading. These tight-fitting doors were better insulated and could maintain an even temperature inside the car. By the mid-1970s, the few remaining ice bunker cars were relegated to "top-ice" service, where crushed ice was applied atop the commodity. Cryogenic refrigeration The Topeka, Kansas shops of the Santa Fe Railway built five experimental refrigerator cars employing liquid nitrogen as the cooling agent in 1965. A mist induced by liquified nitrogen was released throughout the car if the temperature rose above a pre-determined level. Each car carried of refrigerant and could maintain a temperature of minus 20 degrees Fahrenheit (−30 °C). During the 1990s, a few railcar manufacturers experimented with the use of liquid carbon dioxide (CO2) as a cooling agent. The move was in response to rising fuel costs, and was an attempt to eliminate the standard mechanical refrigeration systems that required periodic maintenance. The CO2 system can keep the cargo frozen solid as long as 14 to 16 days. Several hundred "cryogenic" refrigerator cars were placed in service transporting frozen foodstuffs, though they failed to gain wide acceptance (due, in part, to the rising cost of liquid carbon dioxide). Because cryogenic refrigeration is a proven technology and environmentally friendly, the rising price of fuel and the increased availability of carbon dioxide from Kyoto Protocol-induced capturing techniques may lead to a resurgence in cryogenic railcar usage. Experimentation Aluminum and stainless steel Several experimental cars were built when wartime production restrictions were relaxed in 1946: Illinois Central Railroad number 51000 was built in the McComb, Mississippi shops with an aluminum superstructure to reduce weight with steel where required for strength and provided the standard dimensions, cushioned draft gear, easy-riding trucks, minimum of insulation, adjustable ice bunker bulkheads and half-stage icing racks with forced air circulation through side wall flues and floor racks recommended by UFF&VA. Santa Fe Refrigerator Despatch number 13000 was built of stainless steel by the Consolidated Steel Corporation of Wilmington, California with convertible ice bunkers, side ventilation ducts, and axle-driven circulation fans.