The UK’s longest railway bridge, spanning 2.1 miles, crosses roads, lakes, the River Colne, and the Grand Union Canal northwest of London. Trains will reach speeds of 200mph along the viaduct, which is made up of 1,000 individually shaped deck segments supported by 56 piers. Built for HS2, the bridge was fully completed in September 2025, officially surpassing the historic Tay Bridge, which had held the title of the UK’s longest railway bridge since 1887. Costing around £1.6bn and more than a decade in the making, the Colne Valley viaduct will not only carry passengers at high speed but also represent a major milestone in Britain’s railway engineering history. Located near Denham, close to the M25 motorway, the viaduct links Northolt Tunnel under west London and the Chiltern Tunnel to the north. It is the longest of more than 50 major viaducts planned along the HS2 route between London and the West Midlands. Construction began in 2021, with the final giant deck segment lowered into place in September 2024. Since then, engineers have been fitting parapets, installing …