What other towns do you propose serving with London trains? I'd much rather see normal 8 or 9 carriage trains with a roughly similar service to now. Running two separate trains just doubles staffing costs and increases a performance risk with the join. I don't even see much point leaving 5-carriages at Plymouth. Two 5-carriage trains are unlikely to have much more capacity than one 9-carriage one because of the redundant cabs and duplication of catering facilities etc. There are some services through to Penzance which would be fine with 5-carriages but equally there are some which are too busy for a 5-carriage train. Because of platform lengths running two 5-carriage sets is undesirable too as some of the shortest platforms wouldn't fit carriages from both sets on.
One or more of the following, with splits happening at Plymouth, Exeter or even Par or Truro:
- Paighton
- Newquay
- Falmouth
It would then be possible to replace the DMU which currently trundles back a forth to Newquay with a portion of a service from London, which could see a saving on lease costs and track access charges from the existing.
Also some of the services run by the 222's could be run as 7+4 which would mean that the 7 coach portion wouldn't have that many less seats than the current 8 coach HST's. Also, not all services to Cornwall would need to be run by splitting services, so that if passenger levels demanded it then there could be some of the busiest services could be run by 9 coach IEP, which would see a increase in seats available. It would also be possible to increase the frequency of the services so that passenger numbers are spread over more services than at present.
For the smallest of stations, as long as you can get at least one coach from each portion to stop there then with SDO there would be no problem. As stations are rarely that short it is likely that most if not all would be able to take at least 2 coaches from each portion.
Given the passenger flows from such stations tend to be fairly low anyway such problems tend to be highlighted as a reason not to change, without regards to the fact that the same problem occurs at present with the 8+2 configuration of the HST's. As if you had a station which could only take 4 coaches and the HST's stop so that the front 4 coaches fit on the platform that means an up to 4 coach walk for some passengers. If with 222's or IEP's it's the 4 coaches around the join point, there may be one or two passengers who'd have to walk a bit further (i.e. up to 5 coaches, but only if it were two coaches from each portion. As it would be more likely that it was stopped so that it was 3 and 1 with the one coach being from the 4 coach portion) if it was a train of 222's in 7+4 set up, but if it were 5+5 it would be less as it would be up to a 3 coach walk.
However if the seat booking system is designed well, seats could be allocated to passengers to such stations so that an such walk is minimised for the majority of passengers.