was the fifth recorded instance of locomotives designed and constructed in South African workshops after the Natal Government Railways' engine Havelock of 1888, the Class 2C of 1910, the Class 20 of 1935 and the Class ES of 1936. The Class S1 was, however, the first steam locomotive to be home-built in quantity. Characteristics At the time, the Class S1 was the world's most powerful shunting locomotive on Cape gauge and equivalent track, and all the latest developments were incorporated in its design. Since shunting locomotives are designed to operate at slow speeds in tightly curved shunting yards, there was no need for the leading or trailing wheels which are necessary to improve high speed stability on mainline locomotives. Their absence also simplified design and reduced repair costs. In addition, it is desirable to have as large a proportion of the engine weight as possible carried on the coupled wheels to obtain the maximum adhesion. The locomotives had bar frames, Walschaerts valve gear to actuate their diameter piston valves, and were superheated. While their cylinders, driving wheel diameter, frame and many other features were similar to that of the earlier Class S shunting locomotives, their boilers were much larger, being a shortened version of Loubser's specially designed boiler which was used on the Class 12AR. The main difference from the Class 12AR boiler was in their barrel lengths. Each barrel consisted of three courses for the Class 12AR and two courses for the Class S1, the hind courses being identical. In addition, the Class 12AR had a deeper firebox. These Class S1 boilers could also be fitted to the original Class S engines, but no such reboilerings are known to have taken place. The Class S1 boiler was provided with SAR standard boiler fittings, such as a multi-valve steam regulator fitted in the smokebox, a duplex top-feed valve for the delivery of feedwater from the injectors, a soot blower and external exhaust pipe to the smokebox, and a steam turret for the distribution of steam to the various steam-operated mechanisms. The valves for the latter were mounted on the steam turret and were fitted with spindles which extended into the cab from where they were operated by means of handwheels. The locomotive was built with a Watson cab with a slanted front to allow easy access to the firebox cladding which covered the flexible firebox stays and stay caps. The