Construction of Britain’s longest rail bridge completed

The 2.1-mile-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 200mph 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 140-mile route between London and the West Midlands. Mark Wild, HS2 Ltd’s chief executive, is now leading a comprehensive reset of the programme to deliver the railway in the most efficient way possible and for the lowest reasonable cost.

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.”

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.”

The viaduct stretches across the Colne Valley near the M25 motorway and the village of Denham. It sits between the 8.4-mile Northolt Tunnel, which carries HS2 services beneath London, and the 10-mile Chiltern Tunnel.

It is the longest of more than 50 major viaducts being built for HS2.

HS2 services will eventually operate on dedicated tracks between London and Birmingham, before reaching destinations further north via the existing rail network. It will significantly boost capacity on the busiest section of the West Coast Main Line while driving economic growth.