of Baldwin's 4-2-0s in tractive effort by about 63%. However, the frame and driving gear of his locomotive proved to be too rigid for the railroads of the time, which caused Campbell's prototype to be derailment-prone. As the 1840s approached and more American railroads began to experiment with the new locomotive type, the fell out of favor since it was not as able as the to pull a paying load. continued to be built into the 1850s, but their use was restricted to light-duty trains since, by this time, most railroads had found them unsuitable for regular work. In England, for freight work, four-coupled and six-coupled engines were performing well. However, for passenger work the aim was greater speed. Because of the fragility of cast-iron connecting rods, "singles" continued to be used, with the largest driving wheels possible. For some reason, British manufacturers did not take up the idea of mounting the forward wheels on a bogie for some years. There were possibly fears about their stability and with a long rigid frame, greater speed was achieved, albeit at the cost of a very rough ride and damage to the track. The culmination of this approach was seen in the Crampton locomotive where, to make the driving wheels as large as possible, they were mounted behind the firebox. Usage South Africa In 1923, the South African Railways conducted trials with a prototype petrol-paraffin powered road-rail tractor and, in 1924, placed at least two Dutton steam road-rail tractors in service on the new narrow gauge line between Naboomspruit and Singlewood in Transvaal. The petrol-paraffin prototype and one of the latter had a wheel arrangement. The prototype was a modified Dennis tractor which was fitted with a removable bogie between the front wheels to lift them high enough to prevent ground contact. A ball pin on the bogie fit into a socket in the front axle, and the bogie could easily be removed or replaced by running the tractor up a pair of ramps, placed on both sides of the track. The production model was a modified Yorkshire steam tractor, fitted with jacks at the front to allow a separate bogie to be manoeuvred into position underneath the front axle to guide it on the rails. Without the bogie, the vehicle could still be driven on ordinary roads and had the advantage of being able to be detached and run around the