The class 42 or KDL 3 war locomotive was a compromise between the class 52 war locomotive (KDL 1) and the class 44 heavy freight locomotive. It was developed because the class 52 did not always have sufficient power for heavier freight trains, but the axle loads of the bigger sister were too high for many routes. Austria, which was annexed in 1938, and Poland, which was conquered later, were to be the preferred areas of operation.

As a basis, a boiler with the same dimensions as that of the 44 was used and placed on the chassis of the 52. Only two cylinders were used as power plant, which resulted in very rough running when operating at high speeds and when the entire power was called up. The construction of the boiler of the two pre-production models from 1943 corresponded to the Brotan design developed in Austria. Since this did not provide the desired performance, a conventional boiler was used in series production from 1944. Compared to the class 44, the smoke tubes were shortened to save weight.

In use, the typical weaknesses of the war locomotives, which were partly caused by poor quality production, quickly became apparent. Some locomotives had so many defects that they had to be parked damaged after a test run. The closed driver's cab offered protection from the cold in winter, but also led to unbearably high temperatures in summer. In addition, the two jet pumps were installed inside the driver's cab and generated additional heat.

Production took place at several manufacturers in Germany, Austria and Poland, who were combined in the Gemeinschaft Großdeutscher Lokomotivfabriken (Community of Greater German Locomotive Factories). Due to the course of the war, however, the target of initially 8,000 and later 3,300 locomotives could not be achieved. Only 844 examples were completed by the end of the war. After the war, there were not only many examples of this series in Germany, but also in Poland and Austria larger numbers were ready. Some were also scattered in other countries. However, as most of these were in very poor condition, a lot of work had to be done before they could run again.

Simultaneously with the repairs, there were efforts in the post-war period to improve the structural defects of the war locomotives through conversions. In addition, there were still unfinished locomotives in many places, from which further examples were created. Thus the total number rose to 1,061 by 1949. Since there were a large number of steam locomotives that were not so greatly simplified in German-speaking countries, the class 42 did not last very long there. The decommissioning at DB, DR and ÖBB took place in 1962, 1969 and 1968. The last examples, designated as Ty43 there, retired from service in Poland in 1995.