Glenfinnan Viaduct

The West Highland Railway had been built as far as Fort William by the contractors Lucas and Aird, but the West Highland Railway Mallaig Extension (Guarantee) bill was held up in the House of Commons while the Tory and Liberal parties fought over the question of subsidies for public transport. The Act did pass in 1896, by which time Lucas and Aird and their workers had moved south and new contractors were needed. Robert McAlpine & Sons took the work on, with Simpson & Wilson as engineers. The firm was headed by Robert McAlpine, nicknamed "Concrete Bob" for his innovative use of mass concrete, and concrete was used at Glenfinnan because the hard schist of the district was difficult to work. McAlpine's son Robert, then aged 28, took charge of construction, with his younger son Malcolm appointed as his assistant.

Construction of the extension from Fort William to Mallaig began in January 1897. The viaduct was complete enough by October 1898 to be used to carry materials across the valley, and the line opened on 1 April 1901. The structure cost £18,904. Twenty-one semicircular spans of 50 feet take the single track across the River Finnan at a height of 100 feet, and at 416 yards it is the longest concrete railway bridge in Scotland. The mass concrete it is built from carries no metal reinforcement: poured into formwork, typically with a fine aggregate, it is very strong in compression and weak in tension.

A long-established legend attached to the Glenfinnan Viaduct held that a horse had fallen into one of the piers during construction in 1898 or 1899. In 1987 Professor Roland Paxton tested it, inserting a borescope into boreholes drilled in the only two piers at Glenfinnan large enough to accommodate a horse, and failed to find any evidence of one. Acting on local hearsay, he investigated the Loch nan Uamh Viaduct by the same method in 1997, but found its piers full of rubble. Scanning technology used in 2001 located the remains of a horse and cart at Loch nan Uamh, within the large central pylon.

The West Highland Line connects Fort William and Mallaig, and was a crucial artery for the local fishing industry and for a Highland economy that had suffered enormously after the Highland Clearances of the 1800s. Passenger trains over the viaduct are operated by ScotRail between Glasgow Queen Street and Mallaig, using Class 153 and Class 156 diesel multiple units, and in summer West Coast Railways runs The Jacobite steam train along the line, a popular tourist event in the area; the Royal Scotsman also works the route. The viaduct stands at the top of Loch Shiel, overlooking the loch and the Glenfinnan Monument.

The viaduct has been a category A listed building since 5 October 1971. It appears on Scottish banknotes: the 2007 series issued by the Bank of Scotland depicts different bridges in Scotland as examples of Scottish engineering, and the £10 note features the Glenfinnan Viaduct. It has also attracted increased fame beyond Scotland as a filming location for the Harry Potter film series, featuring in four of the films, and is sometimes referred to as "The Harry Potter Bridge". The association has brought hundreds of thousands more tourists each year and a significant strain on the hamlet of Glenfinnan and its vicinity, and after a handful of close calls between pedestrians and trains the British Transport Police had to warn fans not to walk on the viaduct. In 2024, following more than six years of planning, repair work on the structure began, with completion expected in less than 12 months.