The team building the Northolt Tunnel for HS2 have reached a new milestone, completing the construction of 34 cross passages.

Cross passages connect the northbound and southbound tunnel and are a safety feature which allows passengers to cross to the adjacent tunnel in the event of an emergency.

The Northolt Tunnel is 8.4 miles long, running underground between West Ruislip and Victoria Road in Ealing, West London – just outside HS2’s Old Oak Common station. At its deepest point, the tunnel is 35 metres below ground. The tunnel was constructed using four tunnel boring machines (TBMs) and the excavation was finished in June 2025.

Following completion of the cross-passages, engineers will turn to the installation of reinforced concrete secondary collars, which will form the permanent openings for cross passage doors and enable the installation of permanent fire-rated sliding doors.

It is expected that all structural works on the tunnel – the second longest on the 140-mile route between London and the West Midlands – will be completed later this year. The tunnel will then be readied for installation of the rail systems such as tracks and overhead power in the coming years.

Once the team has completed the lined tunnel, a water-proof membrane is installed followed by a secondary concrete lining.

HS2’s London Tunnels Contractor is Skanska Costain STRABAG joint venture (SCS JV). They are responsible for the main works civils construction of HS2 between Hillingdon and Camden, bringing the HS2 line into Euston.

Dave Hannon, tunnels and routeway director from SCS JV, said: “The completion of all 34 cross passages along the Northolt Tunnel marks another important step forward for the programme, demonstrating consistent progress across multiple work fronts and the coordinated effort of our teams working deep beneath London.

“With this phase complete, we can progress on the next stage of works on the tunnel, with safety and productivity at the forefront."

Mark Wild, HS2 chief executive, is currently leading a comprehensive reset of the project to ensure the remainder of the route is delivered as efficiently as possible and for the lowest reasonable cost.