In 1904, the Cape Government Railways placed four Karoo Class 4-6-2 Pacific type passenger steam locomotives in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and classified as Class 5B.
Following on the success of the first two Karoo Class locomotives of the Cape Government Railways (CGR), a further four were ordered from Beyer, Peacock and Company in 1904 and delivered in that same year. In view of the experience gained with the original two Karoo Class locomotives, their design was modified slightly by CGR Chief Locomotive Superintendent H.M. Beatty.
They were numbered in the range from 905 to 908 and, like the previous two locomotives, they were also not allocated class numbers by the CGR. Instead, they were also known as the Karoo Class, from the region of the Western System where they were designed to work.
The type YE1 tender was introduced along with these locomotives. It rode on three axles and had a capacity of 6 long tons (6.1 tonnes) coal and 2,825 imperial gallons (12,800 litres) water.
With these locomotives, Beatty allowed a 1⁄2 inch (13 millimetres) increase in the boiler pitch compared to that of the first two Karoo Class engines, to 7 feet 1 inch (2,159 millimetres) above the railhead.
This increase still did not allow sufficient clearance between the boiler barrel and the 60 inches (1,524 millimetres) diameter coupled wheels. Pockets in the boiler barrel, similar to those used on the earlier locomotives, were therefore still necessary. The boilers of the first two locomotives, numbers 905 and 906, were fitted with Coale patent safety valves, while numbers 907 and 908 had Ramsbottom safety valves. The eccentrics and motion were actuated from the driving (centre) axle instead of the trailing axle. The firebox had an inside width of 4 feet 2+1⁄2 inches (1,283 millimetres).