Network Rail has closed its consultation on a 5km tunnelled rail link between Heathrow Airport and the West Coast mainline.
The proposals, say Network Rail, would improve journeys for millions of air passengers, while also relieving congestion at London Paddington.
Improved public transport access to the Airport from the West should reduce traffic on the A4, M4 and M25 around the Airport area.
Network Rail say the plans do not depend on whether Heathrow is allowed to build another runway. It has been suggested that the tunnel would help create an extra 42000 jobs.
The link would open in 2021.