HS2 launches final giant TBM “Karen” for Euston Tunnel as High-Speed Rail enters final boring phase

HS2 has reached a major tunnelling milestone with the launch of its final tunnel boring machine (TBM), named Karen, marking the start of excavation for the second bore of the Euston Tunnel. The 1,624-tonne machine has begun its 4.5-mile underground journey from Old Oak Common towards central London, completing the final section of bored tunnelling on the high-speed rail link between the West Midlands and the capital.

The Euston Tunnel is a twin-bore structure designed to bring high-speed rail services directly into central London. The first TBM, Madeleine, was launched earlier in 2026, while Karen will excavate the parallel tunnel, completing the full alignment. 

The TBM operates as a fully integrated underground system, excavating soil while simultaneously installing precast concrete segments to form the tunnel lining. Across both tunnel bores, approximately 48,000 segments will be installed, forming structural rings that provide immediate ground support and long-term durability. The machine is expected to advance at an average rate of around 16 metres per day, excavating an estimated 1.5 million tonnes of material over the full drive.

The launch took place from a station box located approximately 23 metres below ground level at Old Oak Common. At its deepest point, the tunnel will reach depths of up to 66 metres beneath London, passing through complex urban geology and beneath existing infrastructure, including sensitive heritage zones such as Kensal Green Cemetery.

A key logistical component supporting the tunnelling operation is the Atlas Road Logistics Tunnel, an 853-metre-long access tunnel completed in 2024. This infrastructure allows for efficient transport of materials, delivery of precast segments, and removal of excavated spoil via conveyor systems. The excavated material is transported to the Willesden logistics hub and redistributed by rail for reuse in construction projects across the UK, significantly reducing road traffic and environmental impact.

The TBM itself was manufactured by Herrenknecht in Germany and assembled underground before launch. Measuring approximately 198 metres in length, it incorporates excavation, spoil handling, segment installation, and support systems within a single continuous operation. Upon completion of the tunnel drive, key components of the machine will be dismantled and retrieved, while the outer shield will remain embedded and grouted within the tunnel structure.

The Euston Tunnel forms a critical part of the HS2 network, enabling direct access to central London and supporting increased rail capacity. The extension of HS2 to Euston is also closely linked to wider urban regeneration plans, with the redevelopment of the Euston area expected to contribute significantly to economic growth, housing, and infrastructure improvements.

With this final TBM launch, HS2 has now deployed all tunnelling machines required for the main route, with several major tunnels already completed, including the Northolt, Chiltern, Long Itchington Wood, and Bromford tunnels. The project now enters its final phase of large-scale underground excavation, bringing the high-speed railway closer to operational delivery.