The contractor will upgrade a 40-mile stretch of the route, from section 6P, 8 and 9 between Cardiff and Bristol Parkway.
The deal follows a successful four month Early Contractor Involvement (ECI) phase with Network Rail to achieve an agreed level of scope, programme and cost.
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More than 300 people will be employed on the project during peak construction. Main works are schedule to begin this month, with completion set 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."
(LM/MH)
