WORK has begun on opening out a crucial Victorian tunnel in London Docklands to carry Crossrail trains. A dock had to be partly drained so that the Connaught Tunnel, which runs underneath, could be rebuilt.

The tunnel under the Royal Victoria Dock near Silvertown was opened in 1878 but last used for passenger trains in December 2006, when the line to North Woolwich was closed. It is the only existing railway tunnel which is being converted for the purposes of Crossrail, which will otherwise run through new structures.

Part of the line between Stratford and North Woolwich has now been converted to carry Docklands Light Railway trains, but the section through the Royal Docks to just short of the former North Woolwich terminus will be needed for the Crossrail branch to Abbey Wood.

Over the last few months a cofferdam measuring 1,300 square metres, around the size of four tennis courts, has been installed to allow a section of the Royal Victoria Dock to be drained so that Crossrail workers could access the tunnel from above. During the draining of around 13 million litres of water, 332 fish were removed and safely replaced elsewhere in the dock.

Crossrail said parts of the tunnel were narrowed during the 1930s so that the dock could be deepened to accommodate larger ships. This work involved the removal of brickwork and the installation of steel segments, and the hole in the crown of the tunnel will allow Crossrail to remove much of this material as the next stage in the tunnel's modernisation.

Work will now continue to open up this hole, which will eventually be approximately 20 metres long and 10 metres wide.

Connaught Tunnel project manager Linda Miller 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.”