Twin-bore tunnels

A twin-bore tunnel is where two parallel tunnels, each containing a single rail track, are constructed using Tunnel Boring Machines (TBMs).

In total, high-speed trains will travel through over 27 miles of twin-bore tunnels – or almost a fifth of the 140-mile route. This means that HS2’s fleet of tunnel boring machines (TBMs) are excavating and building a total of 55 miles of bored tunnel.

• Bromford Tunnel: 3.5 miles (5.8 km)
• Long Itchington Wood Tunnel: 1 mile (1.6km)
• Chiltern Tunnel: 10 miles (16km)
• Northolt Tunnel: 8.4 miles (13.6km)
• Euston Tunnel: 4.5 miles (7.3km)

Most of HS2’s tunnels are being built by TBMs, which have huge rotating cutterheads which excavate on average around 15 metres each day.

The rotating cutter-head at the front of the TBM bores the tunnel, installing the round concrete segments that form the tunnel walls as it goes.

As well as digging through earth and rock, the TBMs build the tunnels themselves. These subterranean moving factories slot and secure thousands of concrete segments into position, creating the cylindrical corridors for the high-speed trains.

Excavation is complete for the 23 miles of deep twin-bore tunnel between Old Oak Common in west London and the railway’s terminus at Birmingham Curzon Street. Construction teams are now mainly focused on internal walkways, ventilation shafts and cross passages.

Our longest and our shortest tunnels – the 10-mile Chiltern tunnel and the 1-mile Long Itchington Wood tunnel – are both structurally complete, with base slabs, cross passages, concrete finishing and walkways installed.

Tunnelling is complete on our second longest tunnel – the 8.4-mile Northolt tunnel – with the last of four TBMs used to dig the tunnel completing its journey on 26 June 2025.  Work is ongoing inside the tunnel on the cross passages, tunnel fit-out and surface structures.

In Birmingham, the tunnelling of the 3.5-mile Bromford tunnel is complete. The team on board the two TBMs worked at depths of up to 40 metres and skilfully navigated TBMs Elizabeth and Mary-Ann under the M6 motorway, key National Grid infrastructure and the meandering River Tame.- which they crossed four times, with a headspace as low as five metres

Preparatory works are underway to launch the final pair of HS2 tunnel boring machines (TBMs) that will build the fifth and final structure – the Euston tunnel – that will take HS2 to Euston in the heart of the capital.

Twin-bore tunnelling progress

Bromford Tunnel, Birmingham

In the Midlands, TBMs Mary Ann and Elizabeth have completed the excavation of the 3.5-mile Bromford Tunnel, between Water Orton in North Warwickshire and Washwood Heath in Birmingham.

Long Itchington Wood Tunnel, Warwickshire

One tunnel boring machine, TBM Dorothy, was used to construct both bores of the 1-mile Long Itchington Wood tunnel.  The first drive was completed in July 2022 and the second drive completed in March 2023.

Chiltern Tunnel, Buckinghamshire

The 10-mile Chiltern Tunnel is the longest tunnel on the HS2 route.  Two tunnel boring machines, ‘Florence’ and ‘Cecilia’, were used to create the tunnel.  The machines completed their historic 10-mile journeys in February and March 2024 respectively, breaking through at the tunnels north portal near South Heath, Buckinghamshire.

Northolt Tunnel, London

In London, four tunnel boring machines (TBMs) were used to create the 8.4-mile Northolt Tunnel. TBM’s Sushila and Caroline dug the five-mile western section, travelling from West Ruislip on the outskirts of the capital to Green Park Way in Greenford, north-west London. Two more machines – ‘Emily’ and ‘Anne’ were used to dig the 3.4-mile eastern section of the tunnel, mining from Victoria Road in Ealing to Green Park Way.

Future tunnel drives

Euston Tunnel, London

Preparatory works are underway to launch HS2’s final pair of giant tunnelling machines.  They will be used to dig the Euston Tunnel – the final 4.5 miles of underground high-speed railway carrying HS2 trains into the heart of London.

The two 190m-long TBMs, ‘Karen’ and ‘Madeleine’, were transported to Old Oak Common in West London in pieces in 2024, before being reassembled on site.  They are now being reassembled at the eastern end of the station, and prepared to launch to Euston next year.