Both tunneling machines were halted late last week after a waste hopper fell against a rail wagon at the Westbourne Park site serving the Western Tunnel Drive.

They both started work again over the weekend but one has now been halted for the rest of the week while investigations continue by Crossrail and main contractor BFK – Bam/Ferrovial/Kier.

Tunneling work has been ongoing since August.

Problems hit the site last Thursday when the hopper tilted.

A Crossrail spokesman told the Enquirer: “During the transportation of excavated material from the tunnels to the freight trains, the hopper, which feeds excavated material into wagons at Westbourne Park, tilted into an individual wagon. No injuries resulted.

“Network Rail closed the adjacent track to our site as a precaution to enable further investigations to be undertaken.

“As is standard procedure, the Health and Safety Executive was advised.”

The failure means not all tunnel waste can currently be loaded directly from a conveyor belt to be transported away from site.

The conveyor system runs from inside the western tunnels, exits at the tunnel portal at Royal Oak and continues along the length of the site through to Westbourne Park where excavated material is loaded directly onto freight trains.

The hopper tilted over at this location meaning direct loading from the conveyor into freight wagons is presently unable to take place.

The rest of the conveyor system is unaffected so excavated material from the western tunnels can continue to be removed by conveyor ahead of stockpiling and transfer by excavator to freight wagons.