The class 66 tank locomotive was developed as one of the last new-build steam locomotives on 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 coupled axle. In addition, it had a lower axle load for use on secondary lines and at the same time a larger coupling 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 direct-contact 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 setting up the driver's cab.

Since the first mainline diesel locomotives suitable for operation on branch lines were ready for use at this time, no further units of the class 66 were built. The two existing engines were soon equipped with push-pull train controls and were mainly used for passenger transport. A third, almost identical locomotive was delivered to the Mülheim Ruhr Valley Railway in 1959. The Bundesbahn later took it over as road number 66 6006. The two examples originally built for the Bundesbahn were retired in 1967 and 1968 and today road number 66 002 can be viewed in the Bochum-Dahlhausen Railway Museum. In a contribution by Ulrich Budde in the forum “Drehscheibe Online” it is credibly described that the latter is actually 66 6006, which was subsequently given the other number.