Civil Engineering August 2021 | Vol 29 No 7

46 August 2021 Civil Engineering Q Q 4 000 to 8 000 tonne/month: Branch lines with construction cost not exceeding £3 500/mile. Q Q 6 000 to 10 000 tonne/month: Branch lines with construction cost not exceeding £6 000/mile. The “missing link” was the 600 to 4 000 tonne/month bracket, because even the cheapest narrow-gauge lines were still more expensive than Dutton’s upper limit of £1 000/mile. To fill the gap, a “pioneer railway” was required to move agricultural produce to the nearest railway station in a cheap, rudimentary fashion. A pioneer railway would also stimulate further agricultural production, hopefully to the point where the increased freight would warrant a more expensive, conventional branch railway. Dutton set out to find a practical form of pioneer railway – a quest that would take him on a journey of almost ten years. THE ROADRAIL CONCEPT The capital cost of a railway line depends largely on bed preparation, namely levelling, compaction, cuttings and embankments. The bed preparation cost is firstly strongly influenced by geometric design standards. The geometric standards for branch lines at the time were a maximum slope of 1:40 and a minimum curve radius of 165 feet (50.3 m). Secondly, the cost depends on the plate-laying standards for sleepers, rails and fasteners. For branch lines, the mass of the rails was limited to a minimum of 35 lb/‌yard (17.4 kg/m). The standards for both bed preparation and platelaying derived from the maximum axle load and manoeuvrability of the locomotive – the requirements for wagons and coaches are less important. The key to cheaper lines, therefore, lay in finding a lighter and more agile alternative to the conventional locomotive. At the time, road tractors, powered by either steam or internal combustion, offered the only alternative. Dutton proposed a train of conventional coaches and wagons running on a narrow-gauge track, drawn by a rubber-wheeled tractor running with its driving wheels on the outside of the rails. The front wheels of the tractor would be replaced with a small bogie running on the track, steering the tractor automatically. Figure 1 shows the first prototype of the “roadrail” system to illustrate its essential features. Dutton backed up his idea with some basic calculations and arguments: Q Q The achievable traction for conven- tional steel wheels on rails was 0.17 times the axle load. For rubber tyres on roads, the comparative figure was 0.67 – significantly higher. Moreover, it was not so much affected by wet or greasy rails. Q Q The rolling resistance of steel wheels on rails was between 0.003 and 0.01 times the axle load; significantly lower than the comparative resistance between 0.03 and 0.1 for rubber wheels on soil. Q Q Taking both traction and resistance into account, the load on the driving wheels of a conventional locomotive would have to be 3.6 times more than the load on the driving wheels of a roadrail tractor to achieve the same drawbar pull before slippage. For the same drawbar pull, a lighter roadrail tractor could utilise much lighter and cheaper railway tracks. Dutton estimated that rails of between 12 lb/‌yard and 16 lb/yard (6.0 kg/m and 7.9 kg/m) would suffice compared to the 35 lb/‌yard rails (17.4 kg/m) required for a regular locomotive. Q Q Roadrail tractors had a differential between the opposing driving wheels, allowing the negotiation of much tighter curves than locomotives with rigid driving axles. Dutton estimated that the minimum curve radius could be reduced from 165 ft to 50 ft (50.3 m to 15.4 m). Q Q A roadrail tractor allowed the fitting of a gearbox between its engine and driving wheels, to be engaged at steeper gradients or lower speeds. By engaging a lower gear at gradients, Dutton estimated that the existing allowable gradient of 1:40 could be increased to 1:20. Figure 2 was taken from one of Dutton’s papers to graphically demonstrate his proposed design standards for roadrail in comparison with the standards for branch lines. These changes, depending on the Figure 2 Graphical comparison of standards between conventional railway and roadrail (Credit : SAR&H Magazine, February 1918, page 107) Figure 1 The roadrail tractor with train on the balloon curve at Canada Junction (Credit: SAR&H Magazine, February 1918, page 105)