If you read what I have said the 222's can provide more seats over the day (if run hourly) over the HST's (which run 9 services a day) to the west country as well as (I didn't mention on this thread but have on others) is that you can get close to the same number of seats as a HST if run as a pair of sets resulting in an 11 coach train (which is basically the same length as a HST), which would then be run on the busiest of services.
This is very different to the WofE services run by SWT's where the units are already run by MU's meaning that there is no "lost" platform length due to Locos. This then means that there would be a noticeable drop in seats if the 222's were to be introduced. As there is also little or no scope to provide extra services during the day, the only way to even keep the number of seats the same over the day would be to provide a LOT more capacity off peak. However, given a lot of the passenger flows are commuters (compared to the GW services, where the flows are spread through the day more evenly), then this would not be all that helpful.
Although my personal preference would be for class 800's to run hourly leading to an increase in seats per train AND seats over the day over the existing (potentially with an increase in services over the existing 9). However the idea with using the 222's is that it would allow GW the opportunity to determine if the larger capacity of the 800's were needed, whilst gaining the journey time improvements which the 222's would bring over the HST's.
Which is why I responded positively to the post about GW looking (favourably) at the idea of using the class 800's.
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We've been round the houses before about your notions on 222s to Cornwall, such as the idea that the Newquay branch is just crying out for frequent trains to and from London...
You say there is "little or no scope to provide extra services during the day". Where? West of Salisbury? West of Yeovil? Are extra services and seats needed west of Salisbury or Yeovil anyway? The pressure is east of Salisbury and from there it's double or quadruple track - admittedly already busy - all the way to Waterloo, so there might just be the scope for the odd extra train there.
222s may not be perfect for Waterloo-Exeter, but then you could well argue that the 159s aren't either, they just happened to be what was available back in the early 1990s. In the world of rolling stock cascades 158/159s are going to be far easier to find suitable alternative work for than any kind of 22x, which, if they run out of intercity options, will have to be put on inter-regional jobs instead, such as Waterloo-Exeter, however much people go on here about how frightful it would be not using them at top speed.
And why it is that it has taken quite so long to get round to the obvious conclusion - that having a common fleet of new express rolling stock for GW services is the only sensible way to go - beats me. The sooner an order for more IEPs is sorted out, the better, preferably as part of the announcement on the new FGW direct award to provide some clarity, even if the trains won't actually arrive until a competitive franchise tender process has finally been held to find a post-2019 GW operator.
No one in their right mind is going to go through the rigmarole of transferring in a fleet of 222s in order to find out if what GW West Country services really need is more Class 800s. By which time Hitachi will have shut down the production line anyway - which is why an order to follow straight on from the current 800/801 orders is the way to go.