The prototype Class RR-56 Mechanical Reefer was introduced in 1955. Based on the previous RR-54 class of refrigerated box cars, the RR-56 class incorporated lessons learned from the older series of cars to build and operate a larger fleet of mechanical refrigerator cars. The RR-56 were classed as all-purpose mechanical temperature controlled cars by the Santa Fe railroad. The fleet numbered 200 cars numbered 2000 - 2188 and 2500 - 2510 in the series. Delivered with SFRD reporting marks, these were changed in 1963 to SFRP but retained their original car numbers.