page_title: Snowdon Mountain Railway
section_title: Rack railway
hierarchical_section_title: Snowdon Mountain Railway / Rack railway
caption_attribution_description: Snowdon Mountain Railway. A typical wheelset on display at Llanberis. This is probably the leading axle, wheels, cranks, and rack wheels off one of the 1923 built engines. 24th July 2005 Photographer - A.M.Hurrell Camera - Fuji FinePix F10 Modified - Trimmed and file size reduced using Adobe Photoshop 5.0LE
context_page_description: The Snowdon Mountain Railway is a narrow gauge rack and pinion mountain railway in Gwynedd, north-west Wales. It is a tourist railway that travels for 4.7 miles from Llanberis to the summit of Snowdon, the highest peak in Wales.
  The SMR is the only public rack and pinion railway in the United Kingdom, and after more than 100 years of operation it remains a popular tourist attraction, carrying more than 140,000 passengers annually. The line is owned and operated by Heritage Great Britain, operators of several other tourist attractions in the United Kingdom.
  The railway is operated in some of the harshest weather conditions in Britain, with services curtailed from reaching the summit in bad weather and remaining closed during the winter from November to mid-March. Single carriage trains are pushed up the mountain by either steam locomotives or diesel locomotives. It has also previously used diesel railcars as multiple units. The railway has two new hybrid locomotives on order, for introduction at the start of the 2020 season.
  The SMR was the inspiration for the fictional Culdee Fell Railway, appearing in the book Mountain Engines, part of The Railway Series written by Reverend W.
context_section_description: The line is built to 800 mm gauge (2 ft 7 ¹⁄₂ in gauge), a gauge it has in common with several other rack railways in Switzerland. 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.
caption: Pinion wheel (centre), running wheels (outside), automatic brake gear (right), rack and gripper rail (centre bottom)
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