Rack Railway
The line has a track gauge common to other mountain railways of 800 mm (2 ft 7 1⁄2 in). The rails are fastened to steel sleepers.
The line uses the Abt rack system devised by Roman Abt, a Swiss locomotive engineer. The system comprises a length of toothed rail (the rack) between the running rails which meshes with a toothed wheel (the pinion) mounted on each rail vehicle's driving axle. The traditional logo for the railway is a pinion ring engaged on a rack bar. At the stations and passing loops, the real items are mounted on steel frames.
The entire railway is fitted with the rack rail. On sidings and around the yard at Llanberis it comprises a single rack bar, but on the running line, and through all the loops up the mountain, two rack bars are used, mounted side by side with their teeth staggered by half a pitch. This is one of the major features of the Abt system, and helps to reduce the shock of the pinions running along the rack. It also ensures the pinion maintains continuous contact with the rack. The joints between rack bars are also staggered and align with the sleepers - each sleeper supports the rack rail as well as the running rails.
The locomotive pinions engage the rack and provide all the traction and braking effort - the wheels are free to revolve on the drive axles, to allow for the inevitable difference between the wheel radius and the effective radius of the pinion. The wheels serve only to support and guide the vehicle; if the pinion were missing, handbrake on and locomotive crank locked, the vehicle would still roll down the mountain. (The wheels are not capable of providing useful adhesion on such a gradient, so this is not the disadvantage it might seem.) The two driving axle pinions on a locomotive are mounted with a half a pitch difference between them. Combined with the half pitch difference in the two rack bars, this feature aims to make the transfer of power more continuous, and thus smooth the hauling of the train. In spite of this, the vehicles still suffer from very high levels of vibration.
The rack bars are machine cut from a special quality of steel: the profile is not symmetrical and the bars must be installed the right way round. Bars tend to fracture between the fixings. When spotted these breaks are marked and then supported with wedges until the bar can be replaced. The pinions consist of an outer ring that is easily replaced. This ring is mounted onto a centre disc, and springs between the two reduce shock loads and allow the small movement needed to accommodate joints and curves. The pinion rings have symmetrical teeth and are turned round to double their working life.
Read more about this topic: Snowdon Mountain Railway
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