photo: Rail Industry Connect/Colne Valley Viaduct
After ten years of planning and four years of construction, the Colne Valley Viaduct now stands as Britain’s longest rail bridge, representing both the ambition and controversy of HS2.
Britain has officially completed civil engineering work on the Colne Valley Viaduct, a 3.3 km structure that is now the longest railway bridge in the UK. The viaduct, part of the controversial GBP 47 billion (EUR 54 billion) HS2 high-speed rail project, overtakes Scotland’s Tay Bridge, which had held the record since 1887.
A Landmark in British Rail Infrastructure
According to BBC, the viaduct is situated near Denham in Buckinghamshire, stretching across roads, lakes, the River Colne, and the Grand Union Canal. Rising 10 metres above land and water, the structure links HS2’s London tunnels in Hillingdon to the Chiltern tunnels, carrying future trains at speeds of up to 200mph.
Rail Industry Connect reported that the viaduct’s gentle curve is composed of 1,000 uniquely-shaped deck segments, each designed to fit seamlessly into the structure. Construction began in March 2021, with the final segment lowered into place in September 2024. Since then, engineers have completed the finishing touches, including noise barriers, waterproofing, drainage, and a structural health monitoring system designed to ensure the viaduct’s long-term safety.
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Engineering Achievement
According to HS2 Media Centre, HS2 Ltd’s senior project manager, Billy Ahluwalia, 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’s project director, also praised the workforce: "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 was delivered by Align JV, a joint venture between Bouygues Travaux Publics, VolkerFitzpatrick, and Sir Robert McAlpine. The contractors faced significant technical challenges, including building over water and ensuring stability across the valley. As noted by BBC, the viaduct is just one of more than 50 major viaducts being built as part of HS2. It sits between the 8.4-mile Northolt Tunnel and the 10-mile Chiltern Tunnel, two of the most complex elements of the project.
The Tay Bridge, which had held the title of Britain’s longest rail bridge since 1887, is now second to the Colne Valley Viaduct. Unlike its 19th-century counterpart, the HS2 structure combines advanced materials, digital monitoring systems, and modern environmental protections.
Financial and Political Debate
While the viaduct is being hailed as an engineering triumph, the broader HS2 project remains politically divisive. The Sun reported that the project has already cost GBP 40.5 billion (EUR 47 billion) up to April 2025, with a further GBP 7.2 billion (EUR 8.3 billion) budgeted for 2025–26, bringing the total close to GBP 48 billion (EUR 56 billion).
The largest portion has gone to civil engineering works, including bridges, tunnels, and embankments, which have consumed over GBP 26 billion (EUR 31 billion) to date. Costs have also included land purchases, consultants, and the now-cancelled northern leg of the project, which alone accounted for more than GBP 2.6 billion (EUR 3.1 billion).
HS2 Ltd has pointed out that it aims to deliver the railway "in the most efficient way" and at the "lowest reasonable cost". However, critics continue to bring up the ballooning budget and question whether the project will deliver value for money. Once operational, HS2 trains will run on dedicated tracks between London and Birmingham, before continuing north on the existing rail network. The project is expected to boost capacity on the West Coast Main Line and stimulate economic growth in regions along the route.