Interesting.
Had a look at the journey time calculator, the GWML (and Heathrow branch) stations with a direct train to Shenfield:
Maidenhead
Slough
Langley
West Drayton
Hayes & Harlington
Southall
Hanwell
West Ealing
Ealing Broadway
Acton Main Line
-------------------------
Heathrow Central
Heathrow T4
This is actually pretty revealing.
We know Reading will get 2tph off-peak and 4tph peak.
Since interchange is shown for Shenfield, we could assume all of these must go to Abbey Wood.
Since Maidenhead is interchange-free for Shenfield, the additional 2tph starting there must go to Shenfield.
This creates a problem though as Iver and Taplow are shown as needing interchange for Shenfield, that would mean their frequency is only 2tph (as if any of the Reading 4tph stopped there, they'd have no interchange).
My educated guess is the Journey Calculator is showing the off-peak situation, and peak trains will supplement it. So this means Reading's 2tph all day does to Abbey Wood, Maidenhead's all day 2tph goes to Shenfield, the peak 2tph service from Reading stops at Iver and Taplow but its Eastern destination is, as yet, unknown.
Heathrow doesn't get extra peak trains, just 6tph consistently all day, so all Heathrow T4's 4tph must be split between Shenfield and Abbey Wood as neither is shown requiring interchange. Heathrow T5 trains all go to Abbey Wood.
Further findings from the Journey Calculator:
- trains to all eastern and western destinations stop at Ealing Broadway, Hayes and Harlington and Southall.
- trains to Heathrow T4 are the only ones stopping at West Ealing and Acton Main Line and Hanwell. All have interchange-free travel to both Eastern destinations. I think we could reasonably assume this is all the same 4tph and Reading/Maidenhead services won't stop at any of these stations.
Further findings from the Crossrail website:
- Slough is listed as having four trains per hour (not 'up to four'). This indicates the peak time Reading trains don't stop here, interestingly.
- West Drayton is listed as having 'up to six' trains per hour. This indicates the peak time Reading trains do stop here.
- Hayes and Harlington is listed as having 'up to ten' trains per hour. This indicates the peak time Reading trains do stop here, but the full service would be 12, not 10. If this anomaly is correct it can only mean 2tph of the Heathrow Terminal 4 services do not stop at H&H.
What this means, if this is all correct, London Reconnections diagram is definitely wrong (although probably just out of date to give them credit), and Geoff Marshall's diagram is also definitely wrong.