was connected to a sleeve around the first driving axle so that any displacement of the bissel bogie in one direction would cause a similar displacement of the leading driving axle in the opposite direction, thereby steering the driving wheels through curves. The outside cylinders drove the third pair of driving wheels while the inside cylinder drove the second pair through a cranked axle, with the cylinder mounted in an inclined position. The three cranks were arranged at an angle of 120 degrees to each other, allowing for the inclination of the inner cylinder. Service The Class 18 was introduced in an attempt to ease problems which were being experienced with increasingly heavy coal trains on the line between Witbank and Germiston, where the Class MF Mallets were considered as being too sluggish and the hauling capacity of the non-articulated fleet was being stretched to the limit. In service, the locomotives disappointed. In spite of the two sets of flangeless coupled wheels and the Krauss-Helmholtz system, the Class 18 experienced excessive flange and tyre wear while the Chief Civil Engineer claimed increased rail wear. The cylinder design was outdated and along with the lightweight motion and rods, contributed to them being uneconomical high-maintenance machines. The complicated valve gear was not robust enough and frequently gave trouble owing to wear and breakage. The result was that the locomotives had to be shopped at under . Both Class 18 locomotives were withdrawn by 1951. After their locomotives were withdrawn from service, the two Type HT tenders had their mechanical stokers removed and their drawgear modified for use with Class 15CA locomotives. They were then redesignated Type KT. Knuckle couplers In 1927, the SAR began to convert the couplers of its Cape Gauge rolling stock from the Johnston link-and-pin coupling system which had been in use since the establishment of the Cape Government Railways in 1873, to AAR knuckle couplers. Judging from contemporary photographs as well as the official SAR Locomotive Diagram Book and the dimensional locomotive drawings as published by Holland, which were for the most part based on the original as-delivered and unmodified loco­motives, the Class 18 locomotives were delivered new with knuckle couplers fitted, as were the Classes GCA, GF, HF and U which also entered service in 1927. Conversion of all rolling stock would take several years and both coupler types could still be seen on rolling stock into