The project is a Network Rail project aimed at easing congestion from Paddington out to Hayes and on the M4/M25.
It will allow passengers and staff to travel in from the West without going further into London and back out. It is not some kind of park and ride aimed at people leaving their cars at Slough or Langley Long term whilst jetting off somewhere.
Taken from NR themselves the benefits include:
- Reduce rail journey times between Reading and Heathrow by delivering a new, faster, frequent, more reliable direct train service to Heathrow with four trains per hour in each direction. All trains would call at Reading and Slough and alternate trains at Twyford and Maidenhead. Journey times could be as short as 26 minutes from Reading and 6 to 7 minutes from Slough.
- Significantly improve rail connectivity to Heathrow from the Thames Valley, South Coast, South West, South Wales and West Midlands.
- Provide an alternative form of transport for passengers and the large number of people who work at the airport who are currently travelling by road.
- Ease congestion on roads, including the M4, M3 and M25 resulting in lower CO2 emissions equivalent to approximately 30 million road miles per year.
- Generate economic growth and new jobs across the Thames Valley and surrounding areas.
- Reduce passenger congestion at London Paddington.
I doubt this will result in an increase in passenger number for LHR, It is more about getting those passengers in from the West more efficiently and reducing congestion outside the airport.