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 noise barriers and completing waterproofing, drainage and a structural health monitoring system.

The work was delivered by Align JV, a joint venture between Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine. 

The main civil engineering phase concluded at the end of 2025, after which work began to install railway systems. Temporary construction areas and the precast factory are now being removed, with the surrounding land being restored to chalk grassland and woodland as part of the HS2 green corridor project.

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Billy Ahluwalia, HS2 Ltd's senior project manager for the viaduct said following the completion of the viaduct in September: “The completion of the viaduct marks more than 10 years of planning, design and construction. I’m immensely proud of the level of dedication and professionalism shown by the whole team to maintain high standards of safety and quality, working over land and water to deliver.

“It’s a remarkable achievement. The viaduct is a structure of international significance - a stunning feat of engineering that will no doubt stand the test of time.”

Coralie Peroux, Align JV project director, said: “I'm immensely proud of the Align team and our supply chain partners, not just for successfully completing the UK's longest rail bridge, but for the remarkable way they delivered it.

"Faced with the technical challenges in design and construction, they harnessed their expertise, worked as an integrated project team, and embraced collaboration. Their efforts have produced a striking HS2 landmark and created a valuable learning legacy for future designers, engineers and construction professionals.”