When the locomotives of the classes T 10 and T 12 had reached their power limits, the T 18 was developed as the last tank locomotive for passenger trains, which was still taken over by the Prussian State Railways. Due to their successful construction, further orders could also be received from other railways at home and abroad. In order to be able to avoid turning the locomotives at the termini and to be able to reach the same speed in both directions, a tank locomotive with a symmetrical wheel arrangement was chosen. Tender locomotives, which could also reach high speeds in reverse, only appeared later, and in the case of tender locomotives with an asymmetrical wheel arrangement, the running smoothness when reversing was usually not entirely convincing.

The resulting T 18 had a leading bogie, three coupled axles with a wheel diameter of 1,650 mm and a trailing bogie. With full supplies, had an operating weight of 105 tons. In order to be able to negotiate tighter curves despite the long wheelbase, the bogies each had a lateral play of 80 mm and the wheel flanges of the middle coupled axle was thinner by 15 mm. The inside of the plate frame was used as a third water tank in addition to the two water tanks on the sides of the boiler. This resulted in relatively large reserves of water and coal for a tank locomotive. The first nine locomotives could reach a top speed of 90 km/h, all others 100 km/h. With an indicated output of 1,140 hp, trains of 350 tonnes were hauled at 90 km/h on the level, and 315 tonnes at 60 km/h on a gradient of 0.6 percent.

The first locomotives were delivered in 1912 by Vulcan in Stettin, others by Henschel and Hanomag. A total of 534 were built, most of which went to the Prussian State Railways and later to the Reichsbahn. In addition, Württemberg received 20, Alsace-Lorraine 27 and the Saar region also 27. Two more were later given to the Eutin-Lübeck railway. The Turkish State Railways were a foreign customer for the T 18.

From 1925 the locomotives became the class 78 and their number only decreased slightly until after the Second World War. In the end, the Bundesbahn took over 424 and the Reichsbahn 53. Some of the former were equipped for push-pull operation, with the regulator being remote-controlled via an electric motor and compressed air. The locomotives remained in service on both railways until the 1970s. Today there are still six remaining, some of which are operational, but some of which are not open to the public.