Chiltern wouldn't be a bad idea in the long term provided the 100mph max running would be fine for 125mph units. However Chiltern seem content with their MK3s and 68s for the foreseeable future (and must say they are excellent rolling stock) and 168s, which again are great units to travel in. If they ever were to receive the 22Xs to replace the loco stock, it would allow the benefit of 168s to run in longer formations. However I still get a feeling Chiltern may try and nudge in and get some of the ex Scotrail 170s.
With faster units it could well be that sections of lines could be investigated as to whether faster line speeds were possible over sensible lengths, possibly without the need for much if any actual physical works.
It is worth remembering that if rail growth continues then there could be need for the Scotrail 170's elsewhere on the network.
The SWT option would be ideal - the 159s will be getting on by that point and the 22Xs would allow a faster journey potentially as they can exploit the higher speeds up to 100mph. However in order to offer the capacity increase they would need to run in 10 cars formations - whilst the 221s and 220s could be formed up to do this in sufficent numbers, it wouldn't really be a idea for the 222s considering there would unlikely be enough to form enough 10 car sets out of 222s alone to replace the 159s and their inability to be coupled up to 220s/221s.
I was assuming that by 2025 (or very soon after) that wires would have been installed between Salisbury and Basingstoke. In doing so it would only require DMU's for the Exeter services, Thus you could provide 1 train ever 2 hours which ran between Exeter and Waterloo (6 sets with 10 coaches, which would require 24 of the 34 sets which make up the 220 fleet) and then 1 train every other hour which ran a shuttle between Exeter and Salisbury (or Westbury once the wires reach Yeovil) which would require a further few units (which could form 4 sets with 7 coaches which would require the remaining 10 sets of the 220 fleet).
Although as the journey time from Waterloo to Exeter is Waterloo and be ready to start again would likely need 8 hours before a unit was ready to go again it would only need 4 of those 6 units, likewise the sets with 7 coaches would have a 5 hour loop (Exeter to Salisbury and back), so would only need 3 units.
I still feel the best method of getting the extra XC capacity is resolving the issue of the 221s being used at WCML - it is particularly ludricous we have 125mph DEMUS running long lengths under wires, where with some additional electric and bi-mode stock, the issue could be resolved.
If XC got all 20 sets of 221's from the ICWC then it would help, in that all the current fleet of 220's could be run semi-permanently paired up (maybe a few edges of the network would see a unit left behind to form the return portion to reduce the total number of units needed) and still probably have enough slack in the system to replace the HST's.
However (assuming the retention or other replacement for the HST's) it would give a total fleet capacity increase of 40%. Given that passenger numbers have doubled in the last 20 years I would suggest that 40% would probably just bring XC to where they should be now and probably wouldn't allow for growth between now and a possible franchise start date of 2019 and certainly doesn't allow for growth to closer to the end of the franchise (circa 2028 - although there will be extra capacity in the form of HS2 phase one from 2026 between Manchester and Birmingham, although this is small section of the XC network and would mostly only benefit those travelling between those two locations so may not impact XC's total passenger numbers very much).
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I think we'll see them replace HST's in Scotland eventually.
Quite possibly, but in what sort of numbers?
Although Scotland will have 27 HST's and there are 27 sets of 222's, I doubt that it will be quite that simple. As when the HST's need replacing Scotrail may not need that many units (due to electrification) but may need longer units (due to passenger growth).
It also depends on what is planned in terms of electrification beyond 2025 (the start of the next franchise) it could be better to keep the HST's for a few more years (circa 2028) and then just replace with EMU's or get bimodal units.
