Vinci has completed the £35M project to bring the disused Connaught Tunnel back into action for Crossrail.
The contractor deepened, strengthened and widened the 550m-long tunnel in East London.
Built in 1878, the tunnel has not been in passenger use since December 2006, but will form part of the Crossrail network from 2018.
Crossrail Connaught Tunnel project manager Linda Miller said: “It’s great to know that, thanks to our work, this 135-year-old tunnel will once again have a vital role to play in London’s transport network.
“Once Crossrail opens in 2018, up to 12 trains an hour will pass through the tunnel beneath the Royal Docks, hugely improving links between southeast London and the rest of the capital.”
Work in the Connaught Tunnel has featured in BBC2’s recent documentary The Fifteen Billion Pound Railway.