The class 66 tank locomotive was developed as one of the last new-build steam locomotives of the Bundesbahn, of which only two were built in 1955. In its basic design, it corresponded to the class 65, but was smaller and had one less driving axle. In addition, it had a lower axle load for use on secondary lines and at the same time a larger driving wheel diameter in order to still be able to reach 100 km/h. On the one hand, it was intended to replace old passenger locomotives such as the Prussian P 8 and T 18, but also freight locomotives such as the T 14. With a combustion chamber boiler, roller bearings and a modern feed water heater, they corresponded to the current state of development. In addition, welding technology was consistently used in their production and the comfort of the crew was also taken into account when designing cab.

Since the first mainline diesel locomotives suitable for operation on branch lines were ready for use at this time, no further class 66 locomotives were built. The two existing locomotives were soon equipped with push-pull controls and were mainly used for passenger service. A third, almost identical locomotive was delivered to the Mülheimer Ruhrtalbahn in 1959. The Bundesbahn later took it over as number 66 6006. The two locomotives originally built for the Bundesbahn were retired in 1967 and 1968 and today 66 002 can be seen in the Bochum-Dahlhausen Railway Museum. There are now indications that the locomotive shown there is in fact the 66 6006 and was only given the Bundesbahn number.