Construction of Britain’s longest rail bridge completed
posted on 16th Oct 2025 09:23
The 3.4 km long structure carries Britain’s new high-speed railway across a series of roads, lakes, the River Colne and the Grand Union Canal, north west of London. Trains will operate at 320 km/h on the viaduct’s gentle curve which is formed of 1,000 uniquely-shaped deck segments.
Construction of the 10 metre-tall viaduct started in March 2021, led by HS2’s main works contractor for this section of the route, Align, a joint venture between Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine. The last of the giant pre-cast deck segments was lowered into place in September 2024. Since then, Align has been putting the finishing touches to the new landmark. This includes installation of parapet sections, noise barriers, a structural health monitoring system and waterproofing and drainage. It means that the Tay Bridge, linking Fife and Dundee, has finally ceded the crown of Britain’s longest rail bridge, a record it had held since 1887.
Although it marks a significant achievement on this section of the railway, far more work is needed to bring civil engineering to the same level of completion elsewhere on the 225 km route between London and the West Midlands. Billy Ahluwalia, HS2 Ltd's senior project manager for the viaduct, said: “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.”
The viaduct stretches across the Colne Valley near the M25 motorway and the village of Denham. It sits between the 13.5 km Northolt Tunnel, which carries HS2 services beneath London, and the 16 km Chiltern Tunnel. It is the longest of more than 50 major viaducts being built for HS2 line. HS2 services will eventually operate on dedicated tracks between London and Birmingham, before reaching destinations further north via the existing rail network.