Summary from English
Wikipedia (enwiki)
The Western Rail Approach to Heathrow is a proposed bi-directional link westward from London's Heathrow Airport to the Great Western Main Line. It would thus run, in council areas, from Greater London under Iver, South Bucks, Buckinghamshire to Langley, Slough. Beginning at Heathrow Terminal 5 station, it would run via a tunnel to a junction east of Langley station, therefore allowing trains to run to and from destinations in the west: Slough, Reading and beyond. When completed, it would improve rail connections to Heathrow from the Thames Valley as well as from South West England, South Wales and The Midlands. It would also reduce congestion at London Paddington station by removing the need, which presently exists, for passengers from Heathrow who are bound for those regions, to travel to Hayes and Harlington and then change at Reading or to make their interchange at Paddington, central London. Paddington would remain an interchange for services to/from Euston, Saint Pancras and London generally and a little of the Great Western Main Line would gain rail capacity as many services would take the resultant side loop via Heathrow.