The Northolt Tunnel, a massive 8.4-mile (13.5 km) underground structure costing £3.3 billion, is set to transform rail travel between London and Birmingham as part of the scaled-down HS2 project. Despite the cancellation of the northern leg by former Prime Minister Rishi Sunak in 2024, this twin-bore tunnel remains a key engineering feat, second only in size on the network.
What the Northolt Tunnel Will Deliver
The tunnel will carry HS2 trains between West Ruislip and the new interchange at Old Oak Common in London, enabling faster journeys to Birmingham. It consists of two separate tunnels: one for northbound and one for southbound services. Tunnel boring machines (TBMs), with rotating cutter-heads that excavate about 15 metres per day, were used to dig the tunnel while simultaneously installing concrete segments to form the cylindrical structure.
HS2 officials noted that TBMs can tunnel beneath towns and cities without disturbing surface activity, and in rural areas, this minimises impact on the countryside. The fourth and final TBM completed its excavation in 2025.
Economic and Construction Milestones
Alan Morris, Construction Delivery Director at HS2 Ltd, stated: “Completing the excavation of this 8.4 mile-long tunnel on HS2 is a real achievement and one the team should be immensely proud of. We’re building HS2 for the future, to increase capacity on our rail network and improve journeys for millions of rail users. The construction of HS2 is already bringing benefits, with £20 billion economic benefit already being delivered at either end of the line.”
In May 2026, the government announced revised cost estimates for completing HS2, ranging from £87.7 billion to £102.7 billion (in 2025 prices). HS2 chief executive Mark Wild described the cost overruns as “terrible news” during a Transport Select Committee hearing, adding that the reasons must be carefully examined.
Political and Future Developments
Transport Secretary Heidi Alexander unveiled a larger cost estimate and extended timeline, with a reset led by Wild scheduled for completion in 2027. Meanwhile, Greater Manchester Mayor Andy Burnham has pledged to reinstate the northern leg of HS2—including Phase 2a (Birmingham to Crewe) and Phase 2b (Manchester and Leeds)—if he becomes Prime Minister.
