Building the entire HS2 route underground using tunnel boring machines would almost certainly have been far more expensive than the mixed approach of tunnels, viaducts, cuttings and surface formation actually planned. Tunnelling is one of the most costly elements of high-speed rail construction. Published HS2 cost guidance shows tunnel unit costs are several times higher per kilometre than surface or viaduct options, with twin-bore tunnel construction estimated at around £22,000 to £25,000 per route metre [1]. The current scheme already incorporates extensive tunnelling—approximately one-fifth of Phase 1 in twin-bore tunnels—precisely where environmental, urban or topographical constraints justify the extra expense. Extending this to the entire 140-mile route would have multiplied these costs substantially.

Full underground construction introduces additional layers of technical and financial complexity beyond raw excavation. High-speed rail tunnels must accommodate aerodynamic pressure waves created by trains travelling at 320 kph, requiring larger cross-sections than standard rail tunnels and increasing excavation volume. Ventilation and emergency-access shafts are required at regular intervals—typically every 2 to 3 kilometres—adding surface land take, vertical excavation costs and mechanical systems. Underground station construction is far costlier than surface or elevated stations. Spoil removal, groundwater management, geological risk contingency and TBM mobilisation lead times would all have extended the programme and raised costs further.

While tunnelling can reduce some land acquisition and legal costs in constrained corridors, these savings are insufficient to offset the massive engineering cost premium of full underground construction. The mixed approach used on HS2—tunnelling where necessary and lower-cost surface or viaduct solutions elsewhere—remains the economically and technically sound option. A fully underground HS2 would likely have pushed the total budget well beyond the current estimate, increased programme risk and complexity, and probably delayed delivery further without eliminating environmental impact or disruption, merely relocating it.