The Snowdon Mountain Railway (SMR) (Welsh: Rheilffordd yr Wyddfa) is a narrow gauge rack and pinion mountain railway in Gwynedd, north-west Wales. It is a tourist railway that travels for 7.5 km. (4.7 m.) 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.

The railway is operated in some of the harshest weather conditions in Britain, with services curtailed from reaching the summit in bad weather. Single carriage trains are pushed up the mountain by a mixture of steam locomotives and diesel locomotives. It has also previously used diesel railcars as multiple units.

The idea of a railway to the summit of Snowdon was first proposed in 1869, when Llanberis was linked to Caernarfon by the London & North Western Railway. No action was taken, as the local land owner, Mr Assheton-Smith, thought a railway would spoil the scenery. A plan to build a railway to the summit of Snowdon from Rhyd Ddu station on the North Wales Narrow Gauge Railways brought fears that Llanberis would lose its income from tourists, and the Snowdon Mountain Tramway and Hotel Company was formed to build the railway. No Act of Parliament was required, as the line was built entirely on private land obtained by the company, without any need for the power of compulsory purchase. This was unusual for a passenger-carrying railway, and also meant that the railway did not come under the jurisdiction of the Board of Trade.

The railway was constructed between December 1894, when the first sod was cut by Enid Assheton-Smith (after whom locomotive No.2 was named), and February 1896, at a total cost of £63,800. By April 1895, it is said that the earthworks were 50% complete, a sign of the effort put into the construction work as much as the lack of major earthworks along much of the route.

All tracklaying had to start from one end of the line, to ensure the rack was correctly aligned, so, although the first locomotives were delivered in July 1895, very little track was laid until August 1895, when the two large viaducts between Llanberis and Waterfall were completed. Progress up the mountain was then quite rapid, with the locomotives being used to move materials as required. Considering the exposed location and possible effects of bad weather, it is surprising that the first train reached the summit in January 1896. As the fencing and signals were not ready, the opening was set for Easter.

The line was opened at Easter 1896. In anticipation of this, Colonel Sir Francis Marindin from the Board of Trade made an unofficial inspection of the line on Friday 27 March. This included a demonstration of the automatic brakes. He declared himself satisfied with the line, but recommended that the wind speed be monitored and recorded, and trains stopped when the wind was too strong.

On Saturday 4 April, a train was run by the contractor consisting of a locomotive and two coaches. On the final section, the ascending train hit a boulder that had fallen from the side of a cutting and several wheels were derailed. The workmen on the train were able to rerail the carriage and the train continued.

The railway was officially opened on Monday 6 April. Two trains were dispatched to the summit. On the first return trip down the mountain, possibly due to the weight of the train, locomotive No.1 "LADAS" with two carriages lost the rack and ran out of control. The locomotive derailed and fell down the mountain. A passenger died due to blood loss from jumping from the carriage. After a miscommunication, a second downward train also hit the carriages of the first, with no fatalities.

An inquiry concluded that the accident had been triggered by post-construction settlement, compounded by excess speed due to the weight of the train. As a result of recommendations of the inquiry, the maximum allowed train weight was reduced to the equivalent of 1½ carriages, leading to lighter carriages being bought and used on two carriage trains. A gripper system was also installed on the rack railway. On 9 April 1897 the line re-opened. This time there were no incidents and the train service continued.

The functioning of the rack and pinion mechanism:

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, comprising a length of toothed rail in between the running rails (the rack), which interfaces with the toothed wheels) mounted on each rail vehicle's driving axle, (the pinion). On sidings and around the yard at Llanberis, the rack rail comprises a single rack bar, but on the running line and through all the loops up the mountain, two rack bars are used, mated side by side, and their teeth staggered by a half a pitch difference. This is one of the major features of the Abt system - it reduces the shock of the pinions running along the rack and 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.

Following the accident in 1896, most of the line was fitted with 'gripper' rails, fixed to either side of the rack rail and of an inverted 'L' cross section. A 'gripper' is fitted to each locomotive, which fits around the gripper rails and holds the locomotive to the rails and prevents the pinion coming free from the rack. Although no other Abt rack railways use a gripper system, other rack systems do.