The class 15A was a 4-8-2 mixed traffic locomotive designed by D.A. Hendrie as a successor of the class 15. It was the last plate frame locomotive of this wheel arrangement designed by him and had the goal to address the overly long tubes of the class 15. The solution was to fit the Belpaire firebox with a combustion chamber that decreased the length of the tubes. This also enlarged the heating surface from 155 to 192 square feet.
The first 13 were built by North British in 1914 with a narrow cab. The later ones got a wider cab and a modified form of the running boards. Additionally, they had no flanges on the first set of driving wheels to improve their running characteristics on lines with sharp curves like the Hex River Pass. In total, North British had built 53 locomotives by 1920. Beyer, Peacock delivered 30 more in 1920 and 1921, followed by 15 more from North British in 1921. The last batch of 21 was delivered by Maffei in 1925.
The first five also had steel fireboxes to decrease the weight on the trailing axle, but these had problems when they were fed with water of poor quality. So their fireboxes were changed to copper and all others were built with copper fireboxes from the start. No. 2100, the last locomotive from the Maffei batch, was built with Lentz rotary cam poppet valves which were later rebuilt to piston valves operated by Walschaerts valve gear, similar to all other class members.
In passenger traffic, the class 15A could reduce the travelling time of the Union Limited by 2.5 hours. In the thirties they became class 15AR when they were fitted with Watson standard No. 2A boilers with round-topped fireboxes and no combustion chambers. Initially they got copper fireboxes, but later rebuilds used steel fireboxes. Although these improved the standardization among the classes of the SAR, but reduced the steaming capacity with a firebox heating surface of only 142 square feet. They also had the distinctive cabs with slanted fronts known from other locomotives. In the early eighties most were used in shunting service, while some were hired out to Swaziland. After their withdrawals few years later, twelve were preserved.
