In order to increase the power of the standard locomotives while at the same time reducing the specific consumption, two medium-pressure test locomotives were built on the basis of the class 03 and designated as class 04. The aim was to run production locomotives with a boiler pressure of 25 instead of 16 bars in the future. The most important change to implement the plans was the use of high-strength steel alloys in the two examples built. The 04 001 was equipped with a boiler made of copper-manganese steel, while the 04 002 got one made of chrome-molybdenum steel. In addition, a compound engine with four cylinders was chosen in order to be able to better utilize the higher steam pressure.
First test runs showed that the power increased from 1,980 hp on the 03 to around 2,320 hp. At the same time, the specific consumption of steam and coal fell significantly, although the 03 was also considered to be very economical. The specific steam consumption was reduced from 6.3 to 5.2 kg per hp hour, while the specific coal consumption based on the drawbar power fell from 1.13 to 0.96 kg per hp hour. Although the boiler itself was able to withstand the higher loads, leaks soon became apparent in the area of the firebox at the seams and stay bolts. Attempts were subsequently made to get this under control by means of rebuilds, but this was unsuccessful. As a result, the boiler pressure was initially reduced to 20 bars and later even to 16 bars.
The two locomotives were now structurally very similar to the prototypes of the somewhat heavier class 02, which is why they were used as 02 101 and 02 102 from now on. With only 2,100 hp, they no longer had any major advantages compared to the 03, which was easier to maintain. The use of other production locomotives from other classes with 20 bars was not convincing either, since the stronger steel alloys showed much more signs of aging than the conventional steels. In the end, all standard locomotives continued to be operated with a boiler pressure of 16 bars. The outbreak of war prevented further promising developments in terms of boiler pressure. There were only isolated attempts with extremely high pressures, but these were also unsuccessful.
