Hello!
Long-time Lurker here.
As a one-time resident of South West London, I have been studying the above with interest and I bring the gift of local knowledge. Tada. So, I've whipped out my old notes (faint sounds of rustling paper) and dusted off an old Google Maps diagram for you amusement (unzipping of pencil case).
Existing and Proposed
www.google.co.uk
Hopefully, that will take you to my diagram of the Airport lines. I've cleaned off the extraneous detail as it was originally done privately to see why the District Line wasn't extended to Heathrow (Answer: Operation and Congestion, but not for discussion here.)
** DISCLAIMER: All the Existing Lines shown are shown have been taken readily available published material **
And apart from that, only the Piccadilly Line from Hounslow to Hatton Cross is shown in any sort of accuracy or detail.
Click the tick boxes for clarity.
The new lines, which I have added to my old diagram, are based on various ideas that have arisen in the past which I have adjusted in the light of site visits, knowledge, etc.
** Piccadilly Line Alternative was for a different route into the central area that was considered and then rejected. I've stuck this in for your amusement.
** SWATH is Southwestern Approach to Heathrow and shows two routes. The route in the far west in pale blue is the T5 route. Note how it hugs the M25 before going below ground at the Horton Road Roundabout. The Site of Special Scientific Interest is clipped at the approach to the Windsor Branch, but this has already been violated by the containers and truck bodies visible when you zoom in to the plan. The line is tucked behind the old Staines West branch earthworks to afford some privacy, although the M25 pretty well destroys that anyway. One refinement is using the old branch line infrastructure south of the Windsor Branch to provide a grade-separated flying junction, although this may be a luxury. Also shown is the avoiding line at Staines. The other route in pale green is one that has popped up from time to time as an alternative and has an intermediate station at Clockhouse. The last version I saw was actually elevated as it crossed the Clockhouse Roundabout. Whoever it was thought this was a good idea... It would involved a very steep incline to the north, but the station would have been interesting. My modification is that the line should dip straight into a tunnel as close to the junction as possible. Somewhere, sometime ago, this was being seriously considered and I have a glossy pamphlet somewhere. But I can't be bothered to look for it now. I have a faint idea that the proposed line also swept through the country park to the west of Bedfont Road in a wide arc. That's a no-no. Personal choice is that the Staines route is the best, in spite of the SSSI.
** WRATH is the well-known and preferred Western Approach to Heathrow, and is tunnelled for much of its length. Construction of the grade-separated junction at Langley should prove interesting. This is more likely to happen, otherwise No Comment.
** And finally, in light red/pink is the Piccadilly Branch to Feltham. This miserable idea kept popping up on various websites and in the local press, suggested by people who should know better and has been doing the rounds for years, and I've put it in for your amusement/despair/tedious groaning and unwanted sighing. It would be single track, in tunnel, with a single underground platform at Feltham and would be operated on an in-out principle, trains coming in from Hatton Cross, reversing and continuing to Terminal 4. Ugh. What a palaver. Rejected. Utterly.
But Wait! What light from yonder window breaks! The Terminal 4 branch of the Heathrow Express/GWR/Elizabeth Line could be extended over the suggested western leg of this proposal to Feltham! It would be single track and there would be a forced interchange with the SWR Windsor/Reading lines. No through running, OHL, one set of points (if at all), and best of all, relatively cheap! That's apart from the horrible ground conditions of layers of gravel, clay and a high water table, but the other subterranean structures in the area have managed. It puts paid to all those problems with level crossings, capacity, and other painful bouts of Extendonitis involving Woking and other such dismal pits of hopelessness. This idea has legs but I'm sticking to Staines. Which sounds worse that it actually is.
Mr. SW.
TLDR Look at the map
