The Glenfinnan Viaduct is a railway viaduct on the West Highland Line in Glenfinnan, Inverness-shire, Scotland, built from 1897 to 1901. Located at the top of Loch Shiel in the West Scottish Highlands, the viaduct overlooks the Glenfinnan Monument and the waters of Loch Shiel.

The West Highland Railway was built to Fort William by Lucas & Aird, but there were delays with the West Highland Railway Mallaig Extension (Guarantee) bill for the Mallaig Extension Railway in the House of Commons as the Tory and Liberal parties fought over the issue of subsidies for public transport. This Act did pass in 1896, by which time Lucas & Aird (and their workers) had moved south.^[1] New contractors were needed and Robert McAlpine & Sons were taken on with Simpson & Wilson as engineers. Robert McAlpine & Sons was headed by Robert McAlpine, nicknamed "Concrete Bob" for his innovative use of mass concrete.^[2] Concrete was used due to the difficulty of working the hard schist in the area.^[3] McAlpine's son Robert, then aged 28, took charge of construction, with his younger son Malcolm appointed as assistant.^[4]

Construction of the extension from Fort William to Mallaig began in January 1897, and the line opened on 1 April 1901.^[5][6] The Glenfinnan Viaduct, however, was complete enough by October 1898 to be used to transport materials across the valley.^[7] It was built at a cost of £18,904.^[3][8]

A long-established legend attached to the Glenfinnan Viaduct was that a horse had fallen into one of the piers during construction in 1898 or 1899.^[9][10] In 1987, Professor Roland Paxton failed to find evidence of a horse at Glenfinnan using a borescope inserted into boreholes in the only two piers large enough to accommodate a horse.^[10] In 1997, on the basis of local hearsay, he investigated the Loch nan Uamh Viaduct by the same method but found the piers to be full of rubble.^[9][10] Using scanning technology in 2001, the remains of the horse and cart were found at Loch nan Uamh, within the large central pylon.^[11]

The viaduct is built from mass concrete, and has 21 semicircular spans of 50 feet (15 m).^[12] It is the longest concrete railway bridge in Scotland at 416 yards (380 m), and crosses the River Finnan at a height of 100 feet (30 m).^[12][8][13] The West Highland Line it carries is single track, and the viaduct is 18 feet (5.5 m) wide between the parapets.^[12] The viaduct is built on a curve of 792 feet (241 m).^[14]

The concrete used in the Glenfinnan Viaduct is mass concrete, which unlike reinforced concrete does not contain any metal reinforcement.^[15] It is formed by pouring concrete, typically using fine aggregate, into formwork, resulting in a material very strong in compression but weak in tension.^[15][16]

The West Highland Line connects Fort William and Mallaig, and was a crucial artery for the local fishing industry and the highlands economy in general, which suffered enormously after the Highland Clearances of the 1800s.

The line is used by passenger trains operated by ScotRail between Glasgow Queen Street and Mallaig, with Class 153 and Class 156 diesel multiple units.^[17] In the summer, West Coast Railways operates The Jacobite steam train along the line. It is a popular tourist event in the area, and the viaduct is one of the major attractions of the line.^[18] The Royal Scotsman also operates on the line.^[19]

Glenfinnan Viaduct has been used as a location in several films and television series, including Ring of Bright Water, Charlotte Gray, Monarch of the Glen, Stone of Destiny, The Crown, and four of the Harry Potter films. After its appearance in Harry Potter, British Transport Police had to warn fans not to walk on the viaduct after a handful of near misses with trains had occurred.^[20] It is also featured in the 2018 video game Forza Horizon 4.

The Glenfinnan Viaduct features on some Scottish banknotes. The 2007 series of notes issued by the Bank of Scotland depicts different bridges in Scotland as examples of Scottish engineering, and the £10 note features the Glenfinnan Viaduct.^[21][22]