The class 01 of the Bulgarian State Railway was the first class in the Bulgarian standardization program. It was a Mikado express locomotive designed for mountainous lines. Like all these standard locomotives, it had many similarities to the German standard locomotives, but had a larger grate due to the lower quality coal available. Furthermore, the boiler was identical with the classes 02, 10 and 46. The 02 was the three-cylinder variant of the 01.

Between 1930 and 1935, 23 were built by manufacturers in Germany, Poland and Switzerland. Initially, they had numbers starting with 8000 and the new class scheme was only introduced in 1937. The Swiss locomotives were called “Tabakloks” since they were delivered in exchange for tobacco.

All 23 locomotives survived World War II. Around 1950, they were converted to run on mazut, what is a low-grade residue from oil production. All locomotives were retired from active service by 1979, with some stored as strategic reserve for some additional time.

The class 11 of the Bulgarian State Railways was a 4-10-0 locomotive for heavy mixed traffic. As a part of the Bulgarian standard program, they were heavily influenced by the German standard locomotives of the Reichsbahn. Like most Bulgarian standard locomotives, they had three cylinders. Thanks to the first driving axle being laterally movable, they could negotiate curves with a radius of 140 meters.

The first ten were built in 1941 and 1942 by Henschel. Two other batches of five and seven were ordered from Škoda and Borsig. Due to relocation of production capacities in favor of the war locomotives, the order for Škoda was cancelled. Thanks to the close relationship between Germany and Bulgaria, the order for Borsig was built anyway.

After only a few years, they were only used in freight service. By 1965, they got a mixed coal and oil firing. Even though they were not very popular with their crews, they were only replaced by diesel locomotives around 1970. In 1975, they were officially retired and scrapped.

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.

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 99^73-76 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.

After the 78 2-6-0 locomotives with 1,470 mm wheels, the Romanian State Railway purchased another series from 1910 onwards, but the wheels only measured 1,350 mm. Although this reduced the top speed, it increased the pulling power. The first 27 Henschel locomotives were delivered with superheaters. However, the other 90 that were delivered by Henschel, Jung and Hanomag up to 1914 were operated with saturated steam. They were given the numbers 1286 to 1427.

During the First World War, five came to the Warsaw Military Railway General Directorate. Four of them became the property of the PKP after the war. They were initially called G5^Rm because they could be classified in the same class as the Prussian G 5 and came from Romania. The name was later changed to Ti102. At least some of the locomotives remaining in Romania were subsequently equipped with a superheater. All of them were scrapped until the 1950s.