Civil Engineering August 2021 | Vol 29 No 7

Civil Engineering August 2021 47 terrain, could reduce the construction cost of a roadrail line to £1 000/mile or less. PILOT TEST AT CANADA JUNCTION By 1917, the theoretical advantages claimed by the roadrail concept con- vinced the SAR&H General Manager, Sir William Hoy, to authorise a “severe” practical test of the concept. A test loop of 678 m was built at Canada Junction (today New Canada), about 15 km to the west of the Johannesburg city centre. The loop was designed to challenge roadrail with all conceivable difficulties – a situ- ation that “would never be required in actual practice”. The rail track was not prepared at all – “crippled and bent” 16 lb/yard rails that had seen “considerable service on construction lines” were laid on 7 lb steel sleepers directly on the dirt found on site, omitting all ballast and checkrails. The alignment was equally severe with the three main curves having radii of 30.5 m, 22.9 m and a balloon curve of 15.2 m – superelevation was deliberately reversed to the opposite direction. The two long straight sections had gradients of 1:17 and 1:18, while the balloon curve was on a gra- dient of 1:18 uncompensated, equivalent to 1:14 compensated. In all respects, the test loop would be more challenging than Dutton’s proposed standards. The layout of the test loop is shown in Figure 3. The rolling stock consisted of a road- rail tractor and three wagons. The tractor, numbered SAR R1501, was a modified Dennis tractor, originally built by Dennis Brothers Ltd in Guildford, England in 1914, powered by a 35 horsepower (26 kW) paraffin engine. It was a regular rubber-tyred road tractor, converted to roadrail by only one change, namely to fit the front axle with a ball socket to lock onto a four-wheeled rail bogie. The bogie thus steered the tractor to automatically follow the railway line. The ball socket also allowed the steering bogie to be attached simply and quickly when the tractor was converted to roadrail use, illustrated in Figure 4. The 3 927 kg mass of the tractor was distributed 35:65 between the steering wheels (front) and the driving wheels (back). The three wagons were second- hand and loaded with wooden sleepers with a total load of 14 500 kg. Instead of hitching the wagons to the tractor, they were hitched directly to the steering bogie Figure 3 The roadrail test loop at Canada Junction (Credit: SAR&H Magazine, February 1918, page 106) Figure 4 The roadrail tractor at Canada Junction being disengaged from its steering bogie (Credit: SAR&H Magazine, February 1918, page 110) Figure 5 SAR&H General Manager William Hoy with Driver Harris at Canada Junction in 1917 (Credit: THL N90074_16a)