The contract award follows a four-month collaboration with Network Rail on an early contractor involvement (ECI) basis to agree scope, programme and cost (which has yet to be disclosed).
Balfour Beatty will be responsible for the remainder of the electrification on the Great Western Mainline route section 6P, 8 and 9 between Cardiff and Bristol Parkway.
Main works are set to start before the end of August, with completion expected towards the end of 2018.
Mark Bullock, managing director of Balfour Beatty’s UK Rail business, said: “Our selection to deliver this important upgrade to the UK rail network builds on our partnership with Network Rail and recognises Balfour Beatty’s capability and expertise.
“Balfour Beatty and Network Rail have successfully delivered the Crossrail West Outer Electrification Programme to schedule and I look forward to passengers seeing the benefits which will come when this new section of electrification is complete.”