As early as 1922, the Bulgarian State Railways had procured tank locomotives of the series 4000 with an 0-12-0 wheel arrangement from Hanomag for transporting the heavy coal trains from Pernik to Sofia. At the end of the twenties, the train performance was to be improved. The challenge now was to transport trains weighing 420 tonnes up the gradient of 2.5 percent at 20 km/h and to negotiate curves with a radius of up to 275 metres. Since it was agreed to achieve a comparable standardization based on the standardization program of the Deutsche Reichsbahn, the boiler had to be interchangeable with the other heavy locomotives which were to be developed new. These were the 8000 series (later 01) 2-8-2 express locomotives and the 9000 series (later 10) 2-10-0 freight locomotive.

In order to be able to negotiate curves with a radius of 250 meters at the appropriate speed with the specified wheel arrangement of 2-12-4T, the running gear had to be designed in a correspondingly complex manner. The only axles which were fixed in the frame and had full flanges were the second and fourth coupled axles. The leading axle was radially adjustable by 80 mm and connected to the first coupled axle to form a Krauss-Helmholtz bogie. There were no wheel flanges on the third and fourth coupled axles. The sixth coupled axle could be moved laterally again and the rear bogie had a lateral play of 110 mm.

The first twelve examples were delivered by the Polish company Cegielski in Poznan in 1931 and actually had a boiler that, together with most of the fittings, could be exchanged with the sister series built by other manufacturers. With 18 cubic meters of water and ten tonnes of coal, the supplies were correspondingly large. The water supply was partly accommodated under the coal bunker at the back in order to be able to keep the side water tanks as short that the firebox was accessible from the sides for maintenance.

Schematic drawing

Die Lokomotive, September 1943

The firebox was over 2.70 meters long in order to have a sufficiently large grate for the low grade Pernik coal with a high proportion of ash and slag. Since this coal led to the formation of a large amount of smoke, this had to be taken into account when passing through the tunnels on the line. The solution consisted of a ventilation system in the cab, which sucked in the relatively clean air from the lower areas and blew it out again at the crew's head height.

The first twelve engines had only two cylinders with a diameter and stroke of 700 mm each. In 1943 another series of eight was ordered after the existing locomotives had already been renamed from the 4500 series to the 46 series. They had to have three cylinders, but despite the placement of the third cylinder under the smoke box, the boiler could not be much higher. The outer cylinders continued to drive the third coupled axle, while the inner cylinder drove the second. Since procurement from Poland was no longer possible due to the war, the order went to BMAG in Berlin.

Since the incline on the route between Pernik and Sofia only permitted 420 tonnes behind such powerful locomotives, the coal trains weighing up to 1,200 tonnes were pulled by up to three locomotives. There was a lead locomotive in front of the actual train locomotive and a pusher locomotive at the end. All having survived until the mid-sixties, they were retired by 1975. One each of the two- and three-cylinder locomotives are still existing today and the latter has been operational again since 2015.