After the three-axle six-cylinder models based on the WR 360 C 14, KHD developed the two-axle models of the T4M series. While four-stroke in-line engines had previously been installed, two-stroke engines in a V design were now used. These were not only lighter than their predecessors, but also saved quite a bit of height. This allowed the hood to be flatter, which significantly improved the driver's view. With 225 hp, the engine of the fifth generation was already more powerful than in the two-axle Wehrmacht locomotive and this was increased again in the sixth generation to between 240 and 260 hp. The use of the same designations for the locomotive and engine soon turned out to be a marketing problem when some T4M 525 R model locomotives were built with engines of the sixth generation. The locomotives were still sold as T4M 525 R, but the engine was now called T4M 625 R and led to confusion with the more powerful locomotive.
Nevertheless, a total of 25 pieces of the weaker and 76 pieces of the stronger variant were sold. Within Germany, six locomotives could again be sold to the British Army of the Rhine, while most customers only bought a smaller number or even individual locomotives. In addition to these locomotives with four-cylinder engines, the three-axle T6M 625 R with 400 hp and the T8M 625 R with 530 hp were also built, in which the same V-engines with six and eight cylinders werde used. With them, the driver's cab was not at the very end, as there was still a smaller hood behind it. Of them, however, only eleven and two pieces were made.
41 of the T4M 625 R found their customers at various operators in Sweden. Most of them ended up with the state railway SJ and were given either the designation Z6 or Z64. Towards the end of the 1970s they not only received 272 hp Scania engines, but also revised and soundproof cabs. Only in the 1990s were the latter transferred to lower-value services and later mostly resold.
