The Cape 6th Class Prairie type locomotive was designed at the Salt River works of the Cape Government Railways and was a further development of the 6th Class Adriatic type. The locomotive itself came about as an experimental development of the very successful 6th Class 4-6-0 locomotive and was initially also built with a Prairie wheel arrangement, but eventually modified to a wheel arrangement.
Development
Good though the 6th Class 4-6-0 locomotives were, the requirement grew for larger and more powerful engines. It was becoming increasingly apparent at the time that such locomotives would require fireboxes with larger grate areas, which was not possible with the existing type of frame. To overcome this problem, CGR Chief Locomotive Superintendent H.M. Beatty prepared designs for a new locomotive with a Prairie wheel arrangement in which the frame terminated in front of the firebox, where it connected to a casting which he termed the "bridle casting". This casting extended out on either side and had wider spaced frames attached to it, which allowed a wider and deeper firebox. Because of the widened frames and larger firebox, trailing carrier wheels were incorporated to take the weight of the firebox. The coupled wheels were also located further forward to accommodate the deeper firebox and, as a result, the leading bogie was replaced by a single axle bissel truck. The first four locomotives of this design, later to be designated Class 6Z by the South African Railways, were placed in service in 1901. They displayed a tendency to be unsteady at speed and the design was therefore modified to a Adriatic type wheel arrangement. Another four locomotives which incorporated this improvement were ordered in 1902, while the first four locomotives were modified accordingly. In spite of their totally different appearance and wheel arrangement, the CGR designated them as 6th Class as well, possibly because they were intended as an improved 6th Class.
Redesign
With an improved design of bissel truck, another two locomotives were ordered from Kitson and Company in 1903 and were once again built with a Prairie wheel arrangement. They had larger boilers as well as larger coupled wheels, compared to the coupled wheels of all other 6th Class locomotives. Numbered 901 and 902 for the Western System of the CGR, these two engines did not display the tendency to sway at speed and therefore retained their wheel arrangement.
When the Union of South Africa was established on 31 May 1910, the three Colonial government railways were united under a single administration to control and administer the railways, ports and harbours of the Union. Although 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 were only implemented with effect from 1 January 1912. In 1912, these two locomotives were renumbered 711 and 712 and designated Class 6Y on the SAR. The rest of the CGR's 6th Class locomotives, together with the Central South African Railways Classes 6-L1 to 6-L3 locomotives which had been inherited from the Oranje-Vrijstaat Gouwermentspoorwegen via the Imperial Military Railways, were grouped into thirteen more sub-classes by the SAR. The 4-6-0 locomotives became SAR Classes 6, 6A to 6H and 6J to 6L, and the eight locomotives became Class 6Z.
Service
In service, these two locomotives proved to be very successful, being fast, powerful, and good steamers. The experience which was gained with these two engines led to the introduction of the Karoo Class 4-6-2 Pacific type later in 1903. The two engines spent practically their entire working lives on the Cape mainline in the Karoo region, until they were withdrawn and scrapped in 1934.
