The £1.6bn railway bridge that’s now UK’s longest | UK | News
The UK’s longest railway bridge, at 3.3 miles long, crosses tracks, lakes, the River Colne and the Grand Union Canal in north-west London. Trains will reach speeds of up to 200 miles per hour along the viaduct, which consists of 1,000 individually shaped deck sections supported by 56 piers. The bridge built for HS2 was fully completed in September 2025, officially surpassing the historic Tay Bridge, which has been the UK’s longest railway bridge since 1887.
Costing around £1.6 billion and taking more than a decade to build, the Colne Valley viaduct will not only carry passengers at high speeds but will also represent a major milestone in Britain’s railway engineering history. Located near Denham, close to the M25 motorway, the viaduct connects the Northolt Tunnel under west London and the Chiltern Tunnel in 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, and the final giant deck segment was lowered into place in September 2024. Since then, engineers have been installing parapets, installing sound barriers and completing the waterproofing, drainage and structural health monitoring system.
The work was carried out by Align JV, a joint venture between Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine.
The main phase of civil engineering was completed at the end of 2025, after which the installation of railway systems began. The temporary construction sites and precast factory are now being removed and the surrounding land is being converted to calcareous pastures 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 am extremely proud of the dedication and professionalism shown by the entire team to maintain high safety and quality standards, working on land and water to achieve these.
“This is a remarkable achievement. The viaduct is a structure of international importance; a dazzling feat of engineering that will undoubtedly stand the test of time.”
Coralie Peroux, Align JV project director, said: “I am immensely proud of the Align team and our supply chain partners for not only successfully completing the UK’s longest rail bridge, but also for delivering it in an outstanding manner.
“When faced with technical challenges in design and construction, they drew on their expertise, worked as an integrated project team and embraced collaboration. Their efforts created a stunning HS2 milestone and created a valuable legacy of learning for future designers, engineers and construction professionals.”
