The £250m job will involve installing overhead electric lines over the 45-mile line, as well as rail infrastructure works at Glasgow Queen Street and Edinburgh Waverley Stations.

Platforms at Croy, Falkirk High, Polmont and Linlithgow will also be extended.

Both firms were awarded a £5m design contract back in January to develop the detailed scope, programme and target price.

The alliance works contract, which is part of the £742m Edinburgh Glasgow Improvement Programme to electrify the route by the end of 2016, is itself almost as big as that for construction of the entire Borders Railway scheme.

Morgan Sindall said its works package would be worth £113m. Under the alliance agreement between Network Rail and Morgan Sindall will both share in end-of-contract losses or profits.

This model of risk-share is designed to create a collaborative environment between all members of the alliance.

Transport Minister, Keith Brown said: “This £250m contract marks a significant milestone in the EGIP project, which will provide 20% quicker journey times, 30% more capacity and a complete overhaul of stations in both cities, with the new-look Haymarket station already completed on time and under budget and the planned transformation of Queen Street station into a 21st century transport hub.”