The contract is the third to be awarded to this JV on the Crossrail scheme.
Works comprise construction of two station box structures, Western Ticket Hall and Eastern Ticket Hall, to provide surface access at either end to twin mined platform tunnels.
Work begins immediately and is scheduled for completion in 2012.
Costain chief executive Andrew Wyllie said: "We are delighted to have been awarded additional work on Crossrail which again demonstrates our broad range of capabilities in the rail sector."
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