Work will now continue to open up a hole that will eventually measure approximately 20m long and 10m wide to strengthen, deepen and widen the central section of the tunnel so that it can accommodate Crossrail’s trains. Work will also include waterproofing, installing water pumps and cleaning the 135 years of coal and soot from the bricks.

Sections of the tunnel are in a poor condition and parts of it were narrowed during the 1930s so that the dock could be deepened to accommodate larger ships with brickwork removed and steel segments installed. The hole in the crown of the tunnel will allow Crossrail to remove much of this material and to continue with the process of ensuring that the tunnel is safe and ready for the arrival of the new trains in 2018.

Over the last few months a cofferdam measuring 1300sqm has been put in place to allow a section of the Royal Victoria Dock to be drained so that Crossrail workers can access the tunnel from above. During the draining of the dock, a total of 332 fish were removed and safely relocated on either side of the cofferdam, Crossrail stated.

The tunnel was built in 1878 and has not been in passenger use since December 2006. It is the only existing tunnel that will be re-used for Crossrail.

Linda Miller, Connaught Tunnel project manager, said: "The Connaught Tunnel is testament to the engineering skill of the Victorians, but after 135 years there’s a lot of work that needs to be done to get it ready for Crossrail. Now we’ve opened the top of the tunnel we’ll start the engineering equivalent of open heart surgery; widening and deepening the structure so that it can accommodate up to twelve trains an hour in each direction."

When the southeast section of Crossrail opens, up to 12 trains an hour in each direction will run through the Connaught Tunnel.