The Württemberg Hz were 0-10-0 rack and adhesion steam locomotives, that were initially developed by the Royal Württemberg State Railways, but were delivered to the Deutsche Reichsbahn-Gesellschaft (DRG) in the mid-1920s.
The locomotives had a Winterthur cogwheel drive with one lower and one upper pair of cylinders. On the level, they ran like normal steam locomotives using the lower, higher pressure, pair of cylinders driving on the third coupled axle. Before entering the rack section the upper drive was started using live steam. At the same time the r.p.m. was matched to the running speed so that entry into the rack section could be achieved smoothly. Once the cogwheel had engaged the rack, the exhaust from the lower cylinders was routed to the upper, lower pressure, ones using a change valve and was expelled from the chimney. The locomotive now worked as a compound.
The cylinders of the adhesion and cogwheel drives have the same diameters ( Ø 560 mm ). The difference in volume (after expulsion from the adhesion system, the steam doubles its volume) is compensated for because the cogwheel drive turns twice as fast. The driving cogwheel is housed in a special frame, that lies above the second and third axle. The higher cogwheel drive and the lower cogwheel are coupled via an intermediate gear with a transmission ratio of 1:2.43.
The boilers were given steel fireboxes and the frame was reinforced, especially in the area of the drive. The outermost axles, which had side play, were given return springs to minimise hunting.