Earlier Natal locomotives were modified from a different original wheel arrangement. The boilers which were used on these locomotives were, at the time, the largest in South Africa. Like the altered NGR Class B of 1906 on which it was based, these loco­motives used saturated steam and had plate frames. They used Walschaerts valve gear with flat "D" gunmetal type valves, which were arranged above the cylinders and controlled by the Hendrie-designed steam reversing gear. Since they were built without superheaters, the bore of their slide valves were larger than those on the altered Class B of 1906 to compensate for the additional weight of the locomotive. The boiler, of which the barrel was thick, was fitted with four Ramsbottom safety valves, in diameter. It was fed by two injectors, arranged on the underside of the footplate at the sides, with the combination valves placed on the back of the firebox. The Belpaire firebox was constructed with a forward "rake" of the throat and back plates to keep the distance between the boiler's tube plates reasonably short at . It had a finger bar firegrate which was operated by hand lever from the cab. When the Union of South Africa was established on 31 May 1910, the three Colonial government railways (Cape Government Railways, NGR and Central South African Railways) were united under a single administration to control and administer the railways, ports and harbours of the Union. Although the South African Railways and Harbours came into existence in 1910 with Sir William Hoy appointed as its first General Manager, the actual classification and renumbering of all the rolling stock of the three constituent railways were only implemented with effect from 1 January 1912. Obsolete locomotives included, altogether 1,460 engines of 70 different types had to be classified and renumbered. Prior to Union, locomotive designs for the constituent railways primarily had to meet local requirements since there had been no through running of engines between them. Union required a more general policy, even though local characteristics such as severity of gradients and curvature and permissible axle loads would never make it possible to produce a standard type of locomotive which would be capable of meeting all requirements. Nevertheless, standardisation was necessary on the new South African Railways (SAR) and Hoy estimated that fifteen standard locomotive types would be sufficient to cover all the varying conditions. In March 1911, one of these new NGR locomotives, no. 368 of the 1911 batch, was used during comparative testing between locomotives of the constituent railways.