Aluminium does not come out of the ground as metal. It comes as bauxite, a clay-like ore whose properties were first understood in 1821. To produce the silver-white metal, you first refine bauxite into alumina by the Bayer process, then reduce alumina to aluminium by the Hall-Héroult process, which is an electrolytic reaction. The catch is that electrolysis consumes enormous amounts of electricity. In the 1890s, that meant hydropower, and in Britain that meant the Highlands. British Aluminium opened the country's first electrolytic smelter at Foyers, on Loch Ness, in 1896. World demand for aluminium rose fast, and a second smelter opened at Kinlochleven in 1908, powered by the Blackwater Reservoir. By 1918 the company wanted to expand further, but Kinlochleven simply did not have the space for the workers' housing the new plant would require. A revised scheme was authorised by the Lochaber Water Power Act of 1921: pipe the water of Loch Treig through 15 miles of tunnel to a brand-new smelter at Fort William.

Construction of stage one began in the mid-1920s. The route ran from a new pier on Loch Linnhe up to the valve shafts at Loch Treig, climbing from 35 feet of altitude to over 800 feet in places. There was no road. There had to be a railway. The consulting engineers, Meik and Halcrow, specified one as a contractual requirement, with a bonus payable to the main contractor Balfour Beatty if the railway and a temporary power station at Monessie Gorge were completed in the first nine months. They earned the bonus. The route presented a series of awkward gradients. Base Camp at the smelter was at 35 feet. Adit 10, the first tunnel access point, was at 640 feet, only 2.5 miles away. A direct line would have required gradients of 1 in 22, far too steep for an adhesion locomotive. The solution was a switchback: 3.25 miles of climbing at 1 in 35 to a junction, then a branch running back along the hillside to the adit. Stretches of 1 in 30 and 1 in 25 were unavoidable. Further along, where the railway crossed peat bogs, the rails were laid on fascines, mats of bundled brushwood. The track flexed under load. The crews learned to live with it.

The genius of the railway was that it let the tunnellers work the 15-mile bore from twenty-three different faces simultaneously. The main tunnel ran from Loch Treig to the smelter. The railway connected base camp to three intermediate shafts, seven adits, and the valve shafts at the loch. Each adit was a side passage where crews could enter the main tunnel and dig in two directions at once. There was also a separate two-foot-gauge railway inside the tunnel itself, used to haul spoil out and lining materials in. At shafts 1, 2, and 7, spoil was lifted out by hoists. The internal tunnel railway eventually grew to about 20 miles total length, almost as long as the surface line, before being removed when the bore was complete. Three teams of men, six to ten in each, worked outward from Base Camp, from the central point, and from Fersit at the upper end, levelling about 30 yards of railway formation per day with pickaxes and shovels. Bridge 15, over the Allt Leachdach, was a 54-foot steel span on concrete abutments. Bridge 27, near Adit 6, was a 40-foot span over the Allt Choimhlidh. Most other bridges were timber trestles, later rebuilt in steel using scrap from the smelter.

Stage one was completed in five years at a cost of £3 million, an enormous sum in the mid-1920s. The permanent pier at the mouth of the River Lochy was finished by the end of 1926, half a mile long, with a three-foot gauge railway running 1.75 miles from the pier through Inverlochy to Base Camp. The pier railway carried alumina in from cargo ships and finished aluminium ingots out, bound for Runcorn on the Manchester Ship Canal. The original route was eventually replaced with a longer one to make way for housing, crossing the West Highland Railway on a plate-girder bridge that still stands. The intention had always been to electrify the permanent pier railway with overhead lines, but it never happened. The Upper Works Railway was retained after construction ended and used for tunnel maintenance for decades. In 1930 the Treig-Laggan branch line was added, crossing the West Highland Railway at Fersit on a 26-span timber trestle that was later filled in to form an embankment. In October 1971, heavy rainfall washed out a 70-foot gap in the Upper Works Railway. Rather than repair the track, British Aluminium and the Forestry Commission agreed to build access roads instead. The line limped on for another six years. It finally closed in 1977. Two locomotives and two speeders were left stranded on the far side of the breach, never recovered. Brazil class 0-4-2ST 'Sir Murray Morrison' was rebuilt by Alan Keef and worked on the Cavan and Leitrim Railway in Ireland from 1994. The bridges are mostly still in place, slowly weathering on the hillside.