HS2 has reached a key milestone on the Northolt Tunnel, with engineers completing all 34 cross passages along the 8.4-mile structure beneath west London.
The cross passages connect the twin tunnel bores and are a critical safety feature, allowing passengers to move between tunnels in the event of an emergency.
Running between West Ruislip and Old Oak Common, the Northolt Tunnel is the second longest on the HS2 route. Excavation of the main tunnel was completed in June 2025 using four tunnel boring machines.
With the cross passages now in place, work will move on to installing reinforced concrete collars to form permanent openings for fire-rated sliding doors. These doors, currently being manufactured in the UK, are designed to withstand high pressure from passing trains and provide fire protection.
Malcolm Codling, Project Client for HS2 Ltd, said the milestone marks steady progress on the scheme.
“Completing the cross passages is an important step towards finishing the tunnel’s civil engineering phase and moving closer to installing track and systems,” he said.
Construction of the passages involved spray concrete lining techniques, with teams excavating between the tunnel bores using mini-excavators and supporting the ground as work progressed. In more complex ground conditions, including areas with high water content, ground freezing methods were used to stabilise the soil before excavation.
The works have been delivered by HS2’s London tunnels contractor, the Skanska Costain STRABAG joint venture, which is responsible for the route section between Hillingdon and Camden.
Dave Hannon, Tunnels and Routeway Director at SCS JV, said the achievement reflects coordinated progress across the project.
“The completion of all cross passages demonstrates consistent delivery across multiple work fronts,” he said. “We can now move forward to the next phase of construction.”
Structural works on the tunnel are expected to be completed later this year, after which the focus will shift to installing railway systems including track and overhead power.
The milestone comes as HS2 continues wider construction across the 140-mile route between London and the West Midlands, alongside ongoing efforts to improve delivery efficiency and control costs.
