Diesel locomotive were classified as DET 1 and DET 2 (). Due to shortage of petrol products during World War I, they remained unused for regular service in Germany. In 1922, they were sold to Swiss Compagnie du Chemin de fer Régional du Val-de-Travers (), where they were used in regular service up to the electrification of the line in 1944. Afterwards, the company kept them in service as boosters till 1965. Fiat claims a first Italian diesel–electric locomotive built in 1922, but little detail is available. A Fiat-TIBB diesel–locomotive "A", of 440CV, is reported to have entered service on the Ferrovie
Diesel locomotive operate efficiently within a limited torque range, and while low power gasoline engines can be coupled to a mechanical transmission, the more powerful diesel engines required the development of new forms of transmission. The first successful diesel engines used diesel–electric transmissions, and by 1925 a small number of diesel locomotives of were in service in the United States. In 1930, Armstrong Whitworth of the United Kingdom delivered two locomotives using Sulzer-designed engines to Buenos Aires Great Southern Railway of Argentina. In 1933, diesel-electric technology developed by Maybach was used propel the DRG Class SVT 877, a high speed intercity two-car