I'm instinctively drawn to the idea of some complicated mega-project, but what destinations would an Interurban service have had? And who would pay for it, given that you'd presumably want electrification of the western legs, since the eastern legs are already electrified? Would it be as far as Ipswich/ Oxford? Or as far as Norwich/ Bristol? Would it be replacing some of the exiting services from London to those places? In which case, what type of stock do you have that accommodates long distance passengers and also can handle large numbers of short distance passengers in central London (given the "churn")? Would Norwich/ Bristol passengers accept something like a 700? Or would you have passengers doing short distance journeys in the "core" dealing with end-doored coaches and a buffet car?
How do you keep a high frequency service in the "core" whilst services are dealing with bottle necks some distance from London?
Essentially, it sounds nice but I think that Heathrow/ Docklands etc will keep the Crossrail services busy enough without the need to put long distance passengers on there too. I can't see any huge "cross London" demand (other than to Heathrow/ Docklands/ Stratford) that requires a longer distance service, beyond the general idea that it'd be nice if there were "something" quirky and long distance.
I see the Reading Crossrail service as Reading-Slough and Maidenhead-London services merged for operational and passenger convenience. Just like it's worth having the even slower Reading-Waterloo service even though it's much faster and more frequent to travel from Reading to Waterloo via Paddington.
^^ that's the way I see it - it's not primarily intended for Reading - London passengers but it makes sense to extend it through to Reading as that's a natural terminus and it can provide Reading (a busy/ important place in its own right, not simply a London commuter town) with a decent level of service/capacity from Thames Valley stations - just like the way that the Waterloo service is really a London - Staines service and a Staines - Reading service joined on together.
Obviously some passengers will prefer a direct train (over a fast Paddington service and trip on the Underground), some passengers will prefer to board an empty service at Reading (instead of cramming onto a service already busy with West Country/Welsh passengers) - horses for courses - it's a busy corridor so there's space for different types of service (and, I have used the Waterloo stopper before, partly because I was going to somewhere on the South Bank and partly because it avoided faffing about changing in central London -
Crossrail *could* have included wires to Reading, but that would have triggered re-signalling and remodelling and thus the costs of those (or a portion of them) would have been loaded onto Crossrail, with relatively few benefits for Crossrail to justify them in itself.
Maidenhead was only ever a terminus of accounting convenience until such a time Reading + Wires was done by somebody else.
Agreed - got to remember that when Crossrail was being planned, the GWML wasn't wired, so there's a danger of scope creep (whereby your project ends up paying for extra things that you'll partly benefit from but other projects will be able to free ride on)
