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
Steam diesel hybrid locomotive a steam engine offered a high starting torque—a tractive force of was available—while a diesel engine offered a high fuel efficiency and it was considered desirable to combine the two. In outline the machine resembled a conventional 2-6-2T steam locomotive, but it had four pairs of horizontally opposed cylinders (13½ inch bore × 15½ inch stroke—343 × 394 mm) mounted longitudinally above the frame and driving a crankshaft placed between them, with the further drivetrain by gears. The cylinders were made double-ended, with the rod end of each cylinder for steam and the closed end fitted with a diesel injector.