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HS2 completes Colne Valley viaduct – UK’s longest rail bridge

By Richard High, 16 September, 2025
An aerial view of The 2.1-mile Colne Valley viaduct, which is now complete and built from 1,000 precast concrete deck segments. The 3.4km-long Colne Valley viaduct is now complete, built from 1,000 precast concrete deck segments.

Civil engineering works are now complete on HS2’s Colne Valley viaduct – the UK’s longest rail bridge – marking a major milestone for the project.

Stretching 3.4km across the Colne Valley near Denham, the structure carries Britain’s new high-speed railway over lakes, the River Colne, the Grand Union Canal and a series of roads. It overtakes the Tay Bridge as the nation’s longest rail bridge, a title held since 1887.

Built by Align JV (Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine), the viaduct is formed of 1,000 uniquely shaped precast deck segments, each manufactured offsite and assembled to create the viaduct’s distinctive gentle curve. The final segment was lowered into place in September 2024, with the past year focused on installing parapet sections, noise barriers, drainage and a structural health monitoring system.

Billy Ahluwalia, HS2’s senior project manager for the viaduct, said: “The completion of the viaduct marks more than 10 years of planning, design and construction. The viaduct is a structure of international significance – a stunning feat of engineering that will no doubt stand the test of time.”

Each deck unit weighed up to 140 tonnes, lifted into place to form the UK’s longest rail bridge. Each deck unit weighed up to 140 tonnes, lifted into place to form the UK’s longest rail bridge.

Coralie Peroux, Align JV project director, added: “Faced with the technical challenges in design and construction, the team harnessed their expertise and collaboration to deliver a striking HS2 landmark and create a valuable learning legacy for future professionals.”

At 10m high, the viaduct sits between HS2’s 13.5km Northolt Tunnel and 16km Chiltern Tunnel. It is the longest of more than 50 major viaducts being delivered for HS2, and will eventually carry trains at up to 360km/h.

Concrete stats – Colne Valley Viaduct

  • Length: 3.4km
  • Height: 10m
  • Segments: 1,000 precast deck units
  • Start of construction: March 2021
  • Completion of last segment: September 2024
  • Completion of civil works: September 2025

HS2 says its construction programme is now undergoing a “reset” under CEO Mark Wild to deliver the railway more efficiently and at the lowest feasible cost. Work has so far started on more than 150 of the 227 bridges and viaducts required across the route.

The 3.4 km Colne Valley Viaduct, the UK’s longest rail bridge, is now structurally complete. The curved structure near Denham will carry HS2 trains at up to 320 km/h and is one of 500 bridges on the project.
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