| Weight | 132t 13cwt |
| Driving Wheels | 4ft 6ins |
| Boiler Pressure | 180psi superheated |
| Cylinders | Outside – 21½in x 28in |
| Tractive Effort | 32,360lbf |
| Valve Gear | Walschaert |
The details above are for the class 4A locomotives as originally built.
These locomotives were designed at Salt River shops as a heavy mixed traffic locomotive by H.M. Beatty, the last Chief Locomotive Superintendent of the Cape Government Railways (CGR).
Soon after the South African Railways (SAR) locomotive renumbering and classification scheme was carried out in 1912, an order was placed with North British Locomotive Company for a further ten locomotives of this type, built to an altered design. They were delivered late in 1913 and were designated class 4A, numbered in the range from 1551 to 1560.
The Class 4A locomotive was an improved version of the predecessor class 4. It had a superheater added, which further resulted in the slide valves and Stephenson valve gear having to be replaced with piston valves and Walschaerts valve gear. The boilers were similar to those of the class 4, except for the superheater and the length between tube-plates, which was 3 inches longer. The engines were erected at the Salt River shops in Cape Town during November 1913. Like their two forerunners, they were excellent steamers and, with the design alterations, gave a much better performance.
The Class 4A were the only locomotives to be delivered with Type XM tenders, which had a coal capacity of 8 long tons and a water capacity of 4,000 imperial gallons.
All ten Class 4A locomotives were reboilered with Watson Standard no. 2 boilers during 1935 and reclassified to class 4AR. In the process the locomotives underwent some additional modifications. The trailing bissel truck remained unaltered, but the side control was redesigned and spring compensation was extended to include the bissel. This was one of the rare instances where reboilering actually appreciably reduced the total weight of the engine, from 85tons 8cwt to 81tons 9cwt.
Their first ten years of service were spent working both passenger and goods trains on various sections of the Cape mainline. At first they worked out of Cape Town, but when more powerful locomotives became available, they were transferred to the Karoo, working between Touws River and Kimberley and also northward from Kimberley to Mafeking. They were then transferred to the Reef, from where they regularly served on the Zeerust, Breyten and Volksrust lines while also being employed in a variety of suburban and local train workings.
During the Second World War SAR 1554 was equipped with temporary protective armour as the locomotive of an armoured train. The locomotive and train were stationed at Mapleton Camp of the Union Defence Force, where the SAR&H Brigade trained before going north to the Middle East.
The last class 4AR locomotive was withdrawn from shunting operations on the West Rand in 1974 after more than 60 years in service. Some remained working in industrial service for several more years, with the last one being finally retired from Apex Colliery in 1983.
Preservation
Two locomotives have survived into preservation.
North British Locomotive Company Works No 20229 – SAR 1555
North British Locomotive Company Works No 20233 – SAR 1560
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Preserved British Steam Locomotives 