When the Otavi Mining and Railway Company in South West Africa needed additional locomotives in 1931, the SAR ordered three class NG15 Mikados from Henschel in Germany. They were based on the Otavi class Hd and SAR class NG5, but were modernized in some aspects and had a larger tender.

A special feature of the new tender was the water tank that extended down to the axles to lower its center of gravity on the narrow gauge lines. As opposed to the earlier locomotives which had a metric gauge of 600 mm, the NG15 was built in two-foot gauge (610 mm). To ensure sufficient running characteristics in curves, the leading axle and the first driving axle were combined with a Krauss-Helmholtz bogie. 

After the first three locomotives, three more were built in 1939. Franco-Belge built ten more between 1950 and 1953. Besides the Otavi, the Tsumeb Copper Corporation got five identical locomotives in 1958. When the Otavi line was converted to cape gauge in 1960, the locomotives were transferred to the Avontuur line in the Western and Eastern Capes. There they got the nickname “Kalahari” due to their former region where they had operated. There they stayed in service even after all other non-Garratt locomotives had been retired.

Their commercial service ended in the late eighties, even though diesel locomotives had been used on the Avontuur since 1973. A huge number of NG15 are existing until today. One of them is operational at the Sandstone Estates in South Africa. In the near future, two are likely to be operational again in Britain on the Ffestiniog and the Welsh Highland Heritage Railway.