The other eighteen locomotives in this order were intercepted by the Imperial Military Railways, who diverted two of them to Lourenço Marques.

In 1912, when the remaining locomotives were assimilated into the South African Railways, they were renumbered again, but retained their Class B designation.

[1][3] In 1891, the Nederlandsche-Zuid-Afrikaansche Spoorweg-Maatschappij (NZASM, often shortened to ZASM) of the Zuid-Afrikaansche Republiek (ZAR) placed an order with Emil Kessler's firm, the Maschinenfabrik Esslingen in Germany, for twenty 40 Tonner 0-6-2 tank steam locomotives.

While these locomotives were satisfactory in service, the trailing wheels initially proved troublesome owing to insufficient sideways freedom of movement when traversing sharp curves.

[1][4][5] To overcome this problem, the next order from the same manufacturer was for 0-6-4T locomotives which were practically identical to the 40 Tonners in their main dimensions, but with a four-wheeled trailing bogie.

[1][5] In 1899, a further order for another twenty 46 Tonners was placed with the Nederlandse Fabriek van Werktuigen en Spoorwegmaterieel (Werkspoor) in the Netherlands.

[1][4] In service, difficulty was experienced with the trailing bogie wheels fouling the firebox and the heads of stays and rivets.

This was partially overcome by fitting stops to the engine frame, but this restriction of the sideways movement of the bogie wheels resulted in derailments in tight curves such as on diverging points.

Cases of derailment on straight track at speed demonstrated the limitations of a design where there was no leading carrying wheel to stabilise the engine and considerable overhang of the cylinders.

[1] The 46 Tonners became the standard mainline locomotives of the NZASM and were used on all kinds of traffic between Pretoria in the ZAR and Lourenço Marques in Mozambique.

On 1 July 1902, when the IMR was transferred to civilian control, the survivors of the NZASM 46 Tonners were taken onto the CSAR roster.

The last 46 Tonner was withdrawn from SAR service and scrapped during 1919, but several had earlier been sold to gold mining companies.

[4][5] Several Class B locomotives were sold to mines and other industries by the CFM, CSAR and SAR when they began to be withdrawn from government railways service.