approximately 183,000. 1931: The SFRD reconfigured seven reefers to utilize dry ice as a cooling agent. 1932: Japanese Government Railways built vehicles specially made for dry ice coolant. 1934: The Interstate Commerce Commission (ICC) regulation #201 came into effect banning billboard advertisements on freight cars. 1936: The first all-steel reefers entered service. 1946: Two experimental aluminum-body refrigerator cars entered service on the PFE; an experimental reefer with a stainless-steel body was built for the SFRD. 1950: The U.S. refrigerator car roster dropped to 127,200. 1955: East of Eden popularizes refrigerator cars by featuring a major plotline where James Dean's father tries to go into the business and fails when all the produce melts during transit. 1957: The last ice bunker refrigerator cars were built. 1958: The first mechanical reefers (utilizing diesel-powered refrigeration units) entered revenue service. 1959: The flush, "plug" style sliding door was introduced as an option, providing a larger door to ease loading and unloading. The tight-fitting doors were better insulated and allowed the car to be maintained at a more even temperature. Early example is the DT&I XL-1 car by Evans. 1966: Japanese National Railways started operation of fish freight express trains by newly built "resa 10000" type refers. 1969: ACF constructed several experimental center flow hopper cars incorporating mechanical cooling systems and insulated cargo cells. The units were intended for shipment of bulk perishables. 1971: The last ice-cooled reefers were retired. 1980: The U.S. refrigerator car roster dropped to 80,000. 1986: The last reefers in Japan were replaced by reefer containers. 1990s: The first cryogenically cooled reefers entered service. 2001: The number of refrigerator cars in the United States bottomed out at approximately 8,000. 2005: The number of reefers in the United States climbs to approximately 25,000, due to significant new refrigerator car orders. 2006 Railex launches 55-car unit train reefer service between U.S. west coast and New York. 2013 Additional unit train reefer services to Florida and Chicago are announced. Specialized applications Express service Standard refrigerated transport is often utilized for goods with less than 14 days of refrigerated "shelf life" — avocados, cut flowers, green leafy vegetables, lettuce, mangoes, meat products, mushrooms, peaches and nectarines, pineapples and papayas, sweet cherries, and tomatoes. "Express" reefers are typically employed in the transport of special perishables: commodities with a refrigerated shelf life of less than seven days, such as human blood, fish, green onions, milk, strawberries, and