ENGINEERS have begun assembling two giant tunnelling machines that will dig the final 4.5 miles to carry HS2 trains underground into the heart of London.

Each of the tunnel boring machines (TBMs) will launch from an underground box at one end of the project’s Old Oak Common station.

They are expected to take around one and a half years to reach the railway’s final southern terminus at London Euston.

In keeping with tradition, the TBMs have been given female names after prominent women in history.

One machine is called Karen after Karen Harrison, the first female train driver in the UK, who was based out of Old Oak Common depot.

The second is named Madeleine, after Madeleine Nobbs, former president of the Women’s Engineering Society.

Unveiling of the TBMs came on the day that Mark Wild joins HS2 Ltd as the company’s new chief executive.

Mr Wild, former CEO of Crossrail, will help oversee the project’s transition to a working railway, with a renewed focus on controlling costs.

The two 190m-long TBMs were built in Germany and transported to Old Oak Common in pieces before being reassembled on site.

This past summer, the HS2 team lifted the machines into the underground station box using a 750-ton crane.

They are now being reassembled at the eastern end of the station, ready to bore to Euston.

The TBMs are like underground factories, excavating the tunnels using a turning cutterhead, 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. Boring is expected to begin in 2025.

Work is already well advanced on the Northolt tunnel – a separate structure running west out of Old Oak Common towards West Ruislip.