HS2 has marked another significant step forward on its London tunnels programme, with the successful completion of all 34 cross passages within the Northolt Tunnel — a critical safety feature on one of the railway’s most complex sections.
The cross passages link the northbound and southbound tunnel bores, providing a safe route for passengers and emergency services to move between tunnels in the event of an incident. Their completion brings the project closer to finishing civil engineering works on what will be the second‑longest tunnel on the 140‑mile HS2 route between London and the West Midlands.
Stretching 8.4 miles, the Northolt Tunnel runs underground from West Ruislip Victoria Road in Ealing to just outside Old Oak Common station in west London. At its deepest point, the tunnel reaches 35 metres below ground. Excavation was completed in June 2025, using four tunnel boring machines (TBMs).
With the cross passages now complete, attention will turn to the installation of reinforced concrete secondary collars. These will form the permanent openings for the cross‑passage doors and allow for the fitting of fire‑rated sliding doors, a vital component of the tunnel’s long‑term safety system.
It is expected that all structural works on the tunnel will be completed later this year, after which the focus will move to preparing the tunnel for rail systems installation, including track, power and overhead line equipment in the coming years.
Malcolm Codling, Project Client for HS2 Ltd, said:
“The progress made to construct the Northolt Tunnel demonstrates the commitment of our teams to deliver this vital infrastructure efficiently and safely.
“Completing the construction of the cross passages is an important step towards finishing the civil engineering on the tunnel which will allow us to ultimately begin rail systems work including laying track.”
Complex engineering beneath west London
The cross passages were constructed using a spray concrete lining (SCL) mining technique, with teams carefully excavating through the ground between the two tunnel bores. The distance between the bores varies significantly, ranging from 6 metres to 20 metres, adding to the technical challenge.
Mini‑excavators were used to advance the excavation, with each metre of ground immediately stabilised using sprayed concrete lining. Once excavation was complete, engineers installed a waterproof membrane, followed by a secondary concrete lining to create the final structure.
In the western section of the tunnel, 11 cross passages were constructed using an advanced ground freezing technique, required due to water‑bearing soils and a high groundwater table. This involved inserting freeze pipes through specially designed tunnel segments to form a two‑metre‑thick frozen ground wall, stabilising the ground before excavation could safely begin.
Industry collaboration delivering results
The works are being delivered by HS2’s London Tunnels Contractor, the Skanska Costain STRABAG joint venture (SCS JV), which is responsible for the main civils works between Hillingdon and Camden, bringing HS2 into Euston.
Dave Hannon, Tunnels & 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."
UK‑manufactured innovation sets new benchmark
The cross‑passage doors for the Northolt Tunnel — and all tunnels across the HS2 route — are being manufactured by Booth Industries in Bolton. Designed to operate in extreme tunnel environments, the doors will withstand constant 14kPa pressure cycles caused by high‑speed trains passing within metres.
They also provide two hours’ fire integrity and two hours’ fire insulation, representing a world‑first and setting a new industry benchmark. The doors are being manufactured using majority UK‑produced steel, supporting domestic supply chains.
Wider HS2 programme continues
While the completion of the Northolt Tunnel cross passages represents clear progress, HS2 acknowledges that substantial work remains across the full route between London and Birmingham. HS2 Ltd Chief Executive Mark Wild is currently leading a comprehensive programme reset to ensure the remaining works are delivered as efficiently as possible and at the lowest reasonable cost.
Image and Video Credits: HS2
