For the narrow gauge railway in the Rhodopes with gradients of three percent and curve radii of 60 meters, the Bulgarian State Railways needed powerful freight locomotives. These were based on the Reichsbahn's class 9973-76 and had slightly larger wheels to reach a speed of 45 km/h. The first five were delivered by BMAG in Berlin, another ten were built in Poland by Fablok due to the Second World War.
They fully met the requirements and were replaced by diesel locomotives of the class 75 in the 1960s. They were now used on other lines with lower gradients, where severe signs of wear showed up due to continuous operation at top speed. After eight locomotives were retired in 1977, the remaining ones were stored as a strategic reserve. Today four still exist, one of which is still operational.