Jacobs is to provide early surveys and design work for a proposed rail link from Langley to Heathrow.
The firm will work for 12 months on the proposed double-track link from the Great Western Main Line to Terminal 5 of the London airport.
The Western Rail Access scheme would improve connectivity from south Wales and the West of England, as well as reduce carbon emissions from road trips to Heathrow.
Jacobs will produce a design for Network Rail’s Development Consent Order, which is expected to be submitted in early 2016.
The firm’s scope of work includes topographical surveys; geotechnical investigations; tunnelling design; and railway systems designs for track and overhead line equipment.
Jacobs Group vice president Bob Duff said: “Jacobs is delighted to continue to leverage our global rail experience on this planned project.
“We look forward to contributing tangible solutions to support Network Rail’s emphasis on sustainable design at the heart of the new rail link.”
Currently in public consultation, if the Western Rail Access is approved, it could be in operation by 2021.