HS2 TBM completes five-mile London tunnel
HS2’s Caroline completes five-mile tunnel under west London
The second of four tunnel boring machines (TBMs) building HS2’s Northolt Tunnel has completed its five-mile (8 km) journey beneath west London, marking a major milestone for the high-speed rail project.
On 3 April 2025, TBM Caroline broke through into a foam concrete-filled reception chamber at Green Park Way in Ealing. Her arrival marks the completion of the western section of the 8.4-mile (13.5 km) twin-bore tunnel, which will carry HS2 trains from West Ruislip to Old Oak Common.
The 2,050-tonne TBM installed 4,217 concrete tunnel rings over the course of her journey, which began in October 2022. Each ring was formed from precast concrete segments, grouted into place as the machine progressed at an average rate of 16 metres per day.
The tunnel’s breakthrough used a high-pressure reception can, designed to manage water ingress and ensure a safe, controlled end to the drive. Once depressurised, the machine will be lifted out for removal.
TBM Caroline, named by pupils at Brentside Primary Academy after astronomer Caroline Herschel, is one of two machines working west to east on the Northolt Tunnel. The first, Sushila, completed her drive in January. Two further TBMs are continuing work on the eastern leg from Victoria Road Crossover Box near Old Oak Common.
Malcolm Codling, client director for HS2 Ltd, said: “We’re immensely proud of the work we have done to complete the second breakthrough for the Northolt Tunnel underneath the capital. We are gearing up for our most productive year to date on this stretch of the line.”
The London tunnels are being delivered by the Skanska Costain STRABAG joint venture (SCS), which is responsible for 12.9 miles (20.8 km) of twin-bore tunnels in total. These include the Northolt and Euston tunnels, along with eight ventilation shafts and headhouses. Excavation of the Euston Tunnel is due to begin within the next 12 months.
Richard Adams, managing director of SCS JV, said: “The arrival of TBM Caroline at Green Park Way marks a significant milestone – the completion of the western section of the Northolt Tunnel – and is a testament to the dedication and skill of all our teams.”
The next phase of work on the completed western section includes the construction of 20 cross-passages, tunnel inverts for track installation, and walkway systems. Cross-passages are a key safety feature, allowing passengers to exit to the opposite bore in an emergency.
Five twin-bore tunnels are being constructed for HS2 between London and the West Midlands. With the 10-mile (16 km) Chiltern Tunnel and 1-mile (1.6 km) Long Itchington Wood Tunnel already complete, 2025 is set to be a key year for TBM progress. Two more machines, Emily and Anne, are expected to complete their drives later this year to finish the Northolt Tunnel.
HS2 is also preparing to launch two new TBMs for the Euston Tunnel, following budget approval in 2024.
Subscribe to the weekly newsletter
