The proposed rail link between Heathrow Airport and Staines aims to connect Terminal 5 to the rail network, providing train services to Waterloo, Guildford and Reading.
Work on the route will take place over the next year, leading to a Transport and Works Act application in early 2009. Temple will help in the consultation process and provide support at a public inquiry if necessary.
Managing director Mark Southwood said: "Our experience of working with key stakeholders and our environmental impact and rail background means that we are confident we can offer the necessary support to BAA and deliver a robust environmental statement."
BAA general rail manager Mike Noakes said: "We were keen to appoint a leading environmental consultancy for Heathrow Airtrack. The scheme is very important."
Temple will work alongside CJ Associates, Halcrow and SKM. The firm's experience includes preparing the sustainability appraisal for Thameslink 2000.