When Crossrail will be open "it will be immediately full"

wouldn't surprise me at all.. commuters on gwml and geml will love not having to change on to underground to reach their final destinations, and people will change from tube at locations like Stratford if crossrail will offer a quicker journey.
it will give some relief to other transport corridors, most significantly the central line (cross rails opening best window for some upgrade works?)

ultimately its going to be full because it will fulfil its design objective of more direct routes, fewer interchanges and faster journeys.

There are a few options that might assist with Crossrail expansion but there is no certainty yet as I think some of the final decisions are yet to be taken.

1. Trains can be extended. I understand the platforms in the central core are designed to take trains that are two carriages longer (12 instead of 10 I think - not 100% certain about train lengths).

2. In line with practice for the Overground transformation I would expect TfL to secure contract options to both extend the Crossrail trains and also to buy far more of them. Clearly funding will be an issue but we talking about 6-7 years hence when hopefully TfL's budget will be a damn sight more stable than now.

3. There was a hint on Twitter a few months ago that Crossrail services into Heathrow would be doubled in frequency. This would bolster service in West London where FGW are already hopelessly overloaded. I expect Crossrail will unleash a torrent of suppressed demand on this corridor meaning more services and trains will be required. Similarly I think people are going to be shocked about the scale of demand on the Abbey Wood branch of Crossrail - it opens first and I expect South Eastern commuters to divert in droves (provided Crossrail is reliable). I also expect travel patterns in SE London to shift hugely with people railheading to Abbey Wood and Woolwich in the way they currently do with North Greenwich.

4. We don't yet know the capability of the signalling system and how many trains it will be capable of handling on the central core. I expect there will be a fair amount of "headroom" to allow more trains to run but clearly you need to have siding and turnback capacity too.

5. There is the long vaunted possibility of extending the Crossrail service to Reading when GW electrification progresses that far. This would give an opportunity to purchase more rolling stock.

The other thing to watch is the extent of off peak travel. If there is little differentiation between peak and off peak demand this may also create pressure for a bigger fleet of trains.

Clearly none of the above is set in stone and speculation on my part. However if Crossrail experiences even half of the demand growth that the Overground has had (a mere 80% growth, the Overground has had 160% growth) then things will have to change pretty quickly. As you say the key is money but provided Crossrail works well then there will be huge stakeholder support from business and politicians for further improvement which should unlock some funding.

Railheading to/from North Greenwich will continue as it is in Zone 2 and Travelcard holders get the bus journey free.

That must be way in the future - probably beyond the end of the next franchise period, unless the trains are to go via the North London Line (reversing somewhere near Gospel Oak to reach the MML)

what are the timescales for re-opening the section between Bicester and Bletchley - and when would it be electrified?

Given the glacial progress being made on connecting the (diesel) lines at Bicester to allow a through service from oxford to Marylebone, I'm not holding my breath that Oxford to Bedford will be possible, let alone by electric train, by the end of the decade

Well if the aim is to see how many people you can fit into Paddington Crossrail station then I'd say that's a pretty good timetable!

Yes.

Aren't they running it in addition to the existing service, which, If I'm correct, will make it
4tph crossrail
4tph heathrow express
4tph heathrow connect

16tph through heathrow central

Right, but who in their right mind would fork out the extra £££ given the Express is essentially the same service and the Connect only saves you around 10 minutes and neither take you any further beyond Paddington!

4+4+4=16?

Concept of Crossrail? Some few years ago I attended a LURS evening meeting when Ken livingstone's then American[?] Tfl director was giving a presentation on Crossrail. He made the point that the idea of CR was not just to get people from one side of London to the other. I questioned him on this having said that surely what was needed to avoid clogging up London mainline termini and LT/NR interchanges was to have an orbital rail version of the M25 so that anyone wanting to go from say, Oxford to Dover [or East Anglia etc] would not have to disembark and trudge across London and add to the transitee mass in central London but could go via a fast orbital route.
He laughed and stated that the point of Crossrail was to get people into central london where they would spend money on food, magazines, taxis etc., whilst waiting for the train to take them on the next part of the journey. It was not being built to make the journey easier/quicker for people who needed to transit London!
The question then arises, "Do we need to have so many CR stations in Central London when there would be less congestion if the stations were only built at NR interchanges?"

arun - if you were serving only Paddington and LST with Crossrail, then you'd hardly need Crossrail - in fact, the Treasury actually tried developing the argument in an effort to cut the project's costs, and even they accepted that the idea wasn't ahem intellectually respectable. In fact, as has been extensively discussed here, the looming problem in central London is a shortage of station capacity and more stations are needed to distribute the traffic, rather than concentrating it on a few stations already near to bursting.

Heathrow Connect is currently 2tph, not 4 (and Heathrow Express is 4tph).

Currently on the Heathrow Connect route we also have the following FGW services:
Southall: 2tph FGW service (so a total of 4tph)
Hayes & Harlington: 4tph FGW (2 x calling at Ealing Broadway but not Southall, 2 x include Southall) (6tph total)
West Ealing: 2tph FGW (from Greenford, also calls at Acton Main Line) (total 4tph)
Ealing Broadway: 6tph FGW service (2 x Greenford, 2 x that call at both H&H and Southall, and 2 x that call at Hayes & Harlington but not Southall) (total 8tph)
Hanwell only has Heathrow Connect call there, while Acton Main Line doesn't and instead has the 2tph Greenford services

@citi - indeed so - the case for new stations to distribute the load is very strong indeed (Note, the capacity of a station is limited by the ability of the escalators to carry the punters to the surface - in a typical double-ended station with three banks of escalators at each end, the limit is 1800 people per bank per hour, so 7200 p/hr in the peak direction - there's a number of stations where that figure is already approached...). If you map the zone 1 areas where the station spacing is thinnest, areas like Belgravia, Mayfair, Fitzrovia and Mount Pleasant stand out. At the risk of being accused of being a closet crayonista, these suggest the usefulness of a s w/ n e routeing through the centre different to that proposed for XR2, which does nothing for station overcrowding/better access.