In the 1930s, the Müglitztalbahn in the Ore Mountains was converted from 750 mm gauge to standard gauge because more and more industrial companies were settling there and the narrow-gauge locomotives were no longer able to cope with the increased demands. Even the traffic on the road did not help. Now there was a need for a tank locomotive that could pull freight trains carrying paper, glass or metal goods. In addition to the maximum gradient of 3.6 percent, the curve radii were a challenge, reaching up to 139 m.
For this purpose, the class 84 was developed as a standard locomotive and initially delivered in two variants with different cylinder layouts. The two examples from BMAG had three cylinders and a chassis with Schwarzkopff-Eckardt bogies. The latter were derived from the well-known Krauss-Helmholtz bogie, but here two driving axles were connected. The carrying axle steered the second coupled axle, which in turn steered the first coupled axle with less deflection. This resulted in a rigid wheelbase of zero.
Two more were delivered by Orenstein and Koppel, which had a Luttermöller axle drive. In this case, the outer driving axles were designed to be laterally movable and connected to the adjacent ones via gear wheels. The carrying axles were mounted in a Bissel frame. Additionally, they had only two cylinders.
In the end, the design of the BMAG was convincing, whereupon eight additional ones were procured. They remained in service on the original line for the time being and eleven of the twelve survived the Second World War, but badly damaged after a bomb attack. After the war, the locomotives remained in the Ore Mountains, where they were used to transport uranium ore, among other things. Their withdrawals took place between 1966 and 1968. Some sources say that all were withdrawn as early as 1958, that a planned rebuild did not materialize due to the small number of units and that they were scrapped after a few years of service.
