Britain’s grand ambition to revolutionize rail travel took another significant leap forward this autumn, as two of the High Speed 2 (HS2) project’s most monumental engineering feats reached completion. The last of the twin tunnel boring machines (TBMs) excavating Birmingham’s Bromford Tunnel broke through on October 13, 2025, while the Colne Valley Viaduct—now the country’s longest rail bridge—was finished with its final deck segment in September 2024. Together, these achievements signal tangible progress on a rail line that has been years in the making and has weathered both technical and political storms.
HS2, Britain’s flagship high-speed railway, is designed to link London with the West Midlands and, eventually, extend further north. For supporters, it represents a once-in-a-generation chance to modernize travel, boost regional economies, and ease pressure on the nation’s aging railways. But the road—or rather, the track—hasn’t always been smooth. Delays, cost overruns, and even a recent construction incident have cast shadows over the project’s timeline and public perception. Still, the recent breakthroughs in tunneling and bridge construction offer a glimpse of the future HS2 promises to deliver.
The Bromford Tunnel, stretching 5.6 kilometers beneath Warwickshire and Birmingham, is set to become the West Midlands’ longest railway tunnel. Its completion marks the end of major tunneling works on the line between London’s Old Oak Common and the West Midlands. The second TBM, named Elizabeth, finished its arduous 19-month journey at Washwood Heath on October 13, 2025. Its twin, Mary Ann, had completed the first bore in May of the same year. These two machines, working in tandem, excavated more than 1.8 million tonnes of Mercia Mudstone and installed 5,804 concrete ring segments, forming the twin eastbound and westbound tunnels.
The engineering challenges faced by the tunneling teams were formidable. The TBMs had to navigate beneath the Park Hall Nature Reserve, burrow under the busy M6 motorway, and cross the meandering River Tame—all while contending with unpredictable and non-homogeneous ground conditions. “It’s been a challenging drive beneath critical live infrastructure and through complex ground conditions,” BBV tunneling director Jules Arlaud told GE. A spokesperson for the tunneling team elaborated earlier this year, explaining, “Its non-homogeneous nature required continual adjustment of TBM parameters. In addition, the variable plasticity of the formation made material extraction during the mining process complex.”
HS2 Ltd’s construction delivery director, Alan Morris, hailed the second breakthrough as a “major milestone for the tunneling team here in Birmingham and for the HS2 project.” Morris added, “All eight of the TBMs digging our tunnels between Old Oak Common and Curzon Street have now broken through, which means that the focus is now on the internal concrete work, ventilation shafts and cross passages. I’m immensely proud of the men and women who have worked round the clock to bring our TBMs and their crews home safely, and I look forward to seeing more progress inside the tunnels in the years ahead.”
While the completion of the Bromford Tunnel is a cause for celebration, it comes against a backdrop of delays and setbacks elsewhere on the HS2 route. On September 29, 2025, a “serious incident” at the Green Park Way vent shafts site in London forced all tunneling work on the London section of HS2 to pause. The Skanska Costain Strabag (SCS) joint venture, responsible for London’s tunnels, has since halted operations as investigations and safety reviews proceed. These delays compound existing challenges, as Transport Secretary Heidi Alexander confirmed earlier in the summer that HS2 services will not commence before 2033.
Amid these hurdles, HS2 Ltd’s chief executive Mark Wild has taken the helm to lead what’s being called a comprehensive reset of the programme. The goal: to deliver the railway in the most efficient way possible and at the lowest reasonable cost. The pressure is on to ensure the project’s next chapters unfold more smoothly, with lessons learned from earlier trials.
Meanwhile, northwest of London, another record-breaking structure has emerged as a new icon of British rail engineering. The Colne Valley Viaduct, stretching 3.4 kilometers and soaring 10 meters above the landscape, is now Britain’s longest rail bridge—surpassing the Tay Bridge, which had held the title since 1887. The viaduct carries HS2’s high-speed trains across a series of roads, lakes, the River Colne, and the Grand Union Canal, all on a gentle curve designed for trains traveling at up to 320 km/h.
Construction of the viaduct began in March 2021, led by the Align joint venture—a partnership between Bouygues Travaux Publics, VolkerFitzpatrick, and Sir Robert McAlpine. Over the course of more than three years, teams installed 1,000 uniquely-shaped pre-cast deck segments. The final piece was lowered into place in September 2024, after which crews focused on finishing touches: parapet sections, noise barriers, a structural health monitoring system, waterproofing, and drainage.
Billy Ahluwalia, HS2 Ltd’s senior project manager for the viaduct, reflected on the accomplishment: “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 is strategically situated near the M25 motorway and the village of Denham, nestled between two other major tunnels: the 13.5 km Northolt Tunnel, which carries HS2 services beneath London, and the 16 km Chiltern Tunnel. It is just one of more than 50 major viaducts being built along the 225 km HS2 route between London and the West Midlands. Eventually, HS2 trains will operate on dedicated tracks between the capital and Birmingham before continuing to northern destinations via the existing rail network.
Despite these engineering triumphs, much work remains before the full vision of HS2 is realized. Civil engineering is still catching up on several sections of the route, and the pause in London’s tunneling underscores the complexity and risk inherent in such a vast infrastructure project. For all the delays and difficulties, the completion of the Bromford Tunnel and Colne Valley Viaduct stands as a testament to the persistence and ingenuity of the teams involved—and a reminder that, even in the face of adversity, the future of British rail is being built, one breakthrough at a time.
With the dust settling on these milestones, the eyes of the nation remain fixed on HS2’s next steps, as Britain inches closer to a new era of high-speed rail travel.
