In World War I, the Halberstadt-Blankenburg Railway in the Harz mountains searched for a way to increase freight traffic on their rack lines which had a gradient of up to six percent. So they ordered two heavy 2-10-2T tank locomotives which could operate with adhesion only and so reach higher speeds. For greater safety when running downhill, they got cog wheels for braking which used the existing rack.

The second and fifth driving axles had side play of 30 mm, while the third one had no wheel flanges. To realize a huge power output, the boiler got a huge diameter of 2,000 mm at a tube length of only 3,700 mm. In the result, these two locomotives had no problems at all with the tight curves and could haul 260 tonnes with 12 km/h on six percent.

Two more locomotives were ordered, but without the braking cog wheels since they were found unnecessary after the trials. These had also been removes from the first two. The four got the names “Mammut”, “Wisent”, “Büffel” and “Elch” (mammoth, bison, buffalo and moose), hence the name “Tierklasse” (animal class).

The Reichsbahn numbered them 95 6676 to 95 6679. Since they needed more powerful ten-coupled tank locomotives to replace slow rack locomotives, the Tierklasse was used as an inspiration for the Prussian T 20. In 1949, all came to the East German Reichsbahn, where the boiler of 99 6679 (ex “Elch”) exploded only two years later. Only 95 6676 (ex “Mammut”) is preserved after its 1968 withdrawal and can today be found non-operational at Dresden.