T is the abbreviation for tank locomotive, ss means that it is a narrow gauge locomotive with a rail gauge of 750 mm (2 ft 5+1⁄2 in), and the letter d, added later, was the abbreviation for duplex locomotive, because the steam was expanded twice, first in the high-pressure cylinders and then in the low-pressure cylinders. Today duplex locomotives are described as compound locomotives.
The engines carried 2.5 m3 (550 imp gal; 660 US gal) of water (the third series had larger water tanks with a 3.0 m3 (660 imp gal; 790 US gal) capacity) and 1.0 tonne (2,200 lb) of coal. The maximal train load was 140 tonnes (140 long tons; 150 short tons) on an incline of 1 in 40 (2.5%).
All the engines were taken over by Deutsche Reichsbahn-Gesellschaft and given the numbers 99 631 to 99 639. After the Second World War four engines were still in service. They were retired as follows:
Number 99 638 – 26 October 1954
Number 99 639 – 27 November 1956
Number 99 637 – 25 March 1965
Number 99 633 – 18 March 1969
Preserved locomotives
Two locomotives, numbers 99 633 and 99 637 have been preserved.
Locomotive 99 633 was under the ownership of the German Railway History Company (DGEG) and was loaned to the Öchsle Schmalspurbahn (Öchsle Narrow-Gauge Railway) and displayed in the Ochsenhausen shed, its original home. In 2007 the society bought it outright. Since the very first transmission of the SWR television programme, Eisenbahn-Romantik, it has featured in the introduction and the programme's logo.
Number 99 637 is on display as a monument at the former station forecourt in Bad Buchau, its last home station.
