The job forms part of a programme to deliver the £650m Edinburgh to Glasgow Improvement Plan .
Last month Network Rail signed alliancing contracts with Costain and Morgan Sindall to develop the detailed scope, programme and target price for EGIP in a deal worth £5m.
This latest deal with see Costain start site works in June on the main Edinburgh-Glasgow route, via Falkirk High.
It includes route clearance works at Winchburgh Tunnel as well as rail infrastructure works at Glasgow Queen Street and Edinburgh Waverley Stations and extensions to intermediate platforms at Croy, Falkirk High, Polmont and Linlithgo.
Andrew Wyllie, chief executive of Costain, said: “We are delighted to have been awarded a further electrification contract by Network Rail as part of an alliance to improve this strategically important transport corridor in Scotland.
“This repeat appointment reflects the success of our approach whereby we form long-term, strategic partnerships with our customers to deliver a broad range of services across the full life cycle of their assets.”