The three-axle tender was unusual, since the leading axle was mounted in a rigid frame while the other two were mounted in a bogie, a design which possibly had the aim of improving stability when running tender-first. This was the first time that a bogie was used under a tender in South Africa, but also the only time that this tender wheel arrangement was used on the CGR. This peculiar tender wheel arrangement appeared in South Africa on only one other occasion, eighteen years later on the Baldwin-built Zululand Railway Company contractor's locomotive in Natal. Like the Neilson-built 4th Class of 1882, these locomotives were built with Joy valve gear, but they had smaller coupled wheels of diameter. The coupled wheels were not spaced equidistant from each other, with a wheelbase of between the leading wheel and the driver and between the driver and the trailing wheel. The locomotives gave good results in service, but were difficult to fire due to their very long and narrow grates. When better quality coal, more suitable for use in the standard locomotives, began to become available from the coalfields in Natal, no more locomotives of this experimental design were ordered. The Joy valve gear was not considered entirely satisfactory. After retired and took over as Chief Locomotive Superintendent of the CGR in 1896, these locomotives were converted to Stephenson valve gear and had their side-tanks removed. By the time the three CGR systems linked up, the system number prefixes had been done away with and these locomotives were renumbered into the range from 641 to 644. By 1904, numbers 641 and 643 no longer appeared in the CGR locomotive register. When the Union of South Africa was established on 31 May 1910, the three Colonial government railways (CGR, Natal Government Railways and Central South African Railways) were united under a single administration to control and administer the railways, ports and harbours of the Union. Even though the South African Railways and Harbours came into existence in 1910, the actual classification and renumbering of all the rolling stock of the three constituent railways was only implemented with effect from 1 January 1912. By 1912, only no. 644 survived. Like the rest of the 4th Class locomotives, it was considered obsolete by the SAR, designated Class 04 and renumbered to 0644. It was scrapped by 1918.