There is no need to shout! How is the train going to get back to the GWML without retracing it's steps?
I am still to be convinced that the benefits are enough to support this shceme. It is a lot of money to pay to take a few coaches off the M4.
As this will all occur after the line has been electrified, trains from Reading can divert off the line East of Slough into Terminal 5.
Terminal 5 station was built with 6 platforms, two for the tube, two for crossrail and two spare. The station box was built for easy connection for a tunnel heading West. The previous Airtrack scheme headed west then south and head Eastwards into Staines. That scheme had a new junction designed so an easy connection could be made to the GWML in a west bound direction.
Current services head East then North up to GWML to Hayes and then to London Paddington. It will be simple to run a local service to Heathrow, trains won't need to reverse as the line would be u shaped. The Eastern leg is already built.
I'm afraid it's not obvious to me. Nor is it obvious how these local services will benefit passengers from Cardiff, Bristol or the West Country.
I am still to be convinced that the benefits are enough to support this shceme. It is a lot of money to pay to take a few coaches off the M4.
The whole M4 corridor has boomed because of it's proximity to Heathrow. There are dozens of mutinational business spread along the corridor. I used to work for one. Connections to the airport are very important and a lot of business travel is reliant on a good connection to the airport. Lots of people also work at the airport, or just around it. witness the morning jams on the M4 for the M25, or traffic coming off junction 5 for the A4.
Good connections here would make a big difference for a lot of people in Reading and points East. 4 trains an hour would in effect become a Thames Valley Metro service.
So no, a cheapish connection for local services won't make a huge difference to Wales other than more convenient connections and a shorter journey time.
But the benefits on a local level will be high and is so much more than a few coaches off the motorway. As to the routing it seems obvious if you look at a map or read the RUS documents at Network rail.
A through service will not require anyone to reverse their train. a service that terminates at Heathrow will not require any reversing to head back west.
We will only be able to judge the true benefits when the actual details are released for consultation.