Whereas the original NWE Mallets have their rear wheels partly concealed, the high-pressure engine at the back of
No. 99.5906 has inside frames. That is just one of the visual differences between the designs, another being the larger cab.
The engine also has a boiler pressure of 12 bar (174lb/sq in) rather than 14 bar (203lb/sq in). However, both designs have a driving wheel diameter of 1m (3ft 3in).
The condition of No. 99.5906 has evolved over time, but although it looks different the Harz engine is classmate to another survivor, No. 105, at Switzerland’s Blonay-Chamby museum line.
In contrast to No. 99.5906, the other surviving Harz Mallets were built by Jung. They are Nos. 99.5901 (1897), 99.5902 (1898) and 99.5903 (1901). Of those, No. 99.5903 is in long-term store.