HS2’s tunnelling team are celebrating completing half of the 8.4-mile twin bored Northolt Tunnel between West Ruislip and Old Oak Common.

Four boring machines (TBMs) are being used to excavate 16.8 miles of tunnel overall – two bores over the 8.4-mile length – making it the second longest tunnel on the new railway after one recently excavated beneath the Chiltern Hills.

The first machine, named Sushila after a local teacher, was launched eastwards from West Ruislip in October 2022.

This was followed one month later by TBM Caroline, named after astrologer Caroline Herschel, which is used to excavate the adjacent tunnel.

Both will complete five miles of the route, finishing their journeys at the Green Park Way vent shaft in Greenford – just over half way – where they will be lifted out.

In February this year, TBM Emily, which is named after Emily Sophia Taylor, began its journey west from HS2’s Victoria Road Crossover Box site near Old Oak Common towards Greenford.

She was joined by the fourth machine, named Anne, after Lady Anne Byron, in May. They will both complete 3.4 miles of the tunnel.

Combined, the four machines have now travelled more than 8.4 miles, half of the total distance for the twin-bored tunnel.

Each machine operates like an underground factory, excavating the tunnels, lining them with pre-cast concrete tunnel segments, grouting them into place before moving forward at an average speed of 16 metres per day.

Teams work around the clock below ground on the TBMs, along with colleagues on the surface supporting them.

As the TBMs progress, other vital features of the tunnel are put in place, such as cross passages, vent shafts and headhouses.

Between them, the quartet of machines have excavated nearly two million tons of earth.

Around the West Ruislip portal, excavated material is being used to re-model a nearby golf course that has been impacted by HS2’s tunnelling.

Two of HS2’s tunnels are now complete – one stretching for 10 miles under the Chiltern Hills and another one mile in length beneath in Warwickshire.

Preparations to bore the final tunnel between Old Oak Common and Euston are underway with two further TBMs due to arrive in the UK this summer.