As you acknowledge, there is overcrowding on GW lines, so you can't afford to then waste capacity by filling the paths with lots of short trains, however clever the signalling may eventually be. And it's no answer to the question of passenger loadings east/west of Oxford on Cotswold services - to name the obvious instance where coupling/uncoupling is expected much of the time - in a situation where train paths between Oxford and Didcot will also be in high demand in future to accommodate extra freight and East-West services on top of all the existing services.
The route is not full though, and once the ERTMS is fitted there will likely be plenty of spare paths to run additional Oxford terminators and the like if there is really insufficient capacity on the through services.
If there is no shortage of paths running 1 5-car through and 9-car Oxford terminator is clearly superior to running 1 5-car through and 1-5 car Oxford terminator, as you only require one additional staff member and have far more seats over the Oxford section of the run for a same outlay in catering staff.
If there was through gangways you would only need one catering crew for the joined up formation to provide the service to path sets, but since there will be no gangways you would require a second crew and only save the driver.
The choke point on the Great Western Main Line is between London and Maidenhead after Crossrail reduces it to a two track railway in that area. There are multiple alternatives for cross london freight that avoid having to use that section of track, and as far as I know little freight currently goes that way anyway.
With the commitment of the Electric Spine it is also more likely than ever that the freight will be electric operated and thus will be far more spritely than most freight services.
Lots of other rolling stock of similar vintage to Turbos, eg 158s, is going to need money spending on it to meet DDA and it is is obvious that the 165/166 fleet will be moving west come 2016/17 so the gauge clearance has to happen before DDA deadlines.
Is it? I don't think it has been decided if the 158s will recieve DDA rebuilds, all of the Sprinter and Networker families will be over, or approaching, 30 years old as you yourself acknowledge and will thus be approaching end of life.
Will it be worth spending several hundred thousand pounds to rebuild trains that are going to see another decade at best and at the same time use engines which are awfully polluting by modern standards? A re-engining programme will put the cost up even further.
Are class 16xs not cleared on the Chiltern/GC routes anyway? They could easily be put to work for a few years on the Chiltern services which require additional stock, perhaps displacing the 172s. Until such time as they are either replaced with new diesels or electrification renders them unneccesary.
where do we start... see other threads here about their general nastiness. HSTs may not be DDA-complaint but at least they don't shake themselves to bits on occasion and the toilets don't reek - or waste acres of precious space in the case of all those disabled toilets in Voyagers. I suggested nothing but you seem to be suggesting we should scrap 16xs due to DDA before their 30th birthdays, so why not 20-odd year-old 22xs as well, even if general nastiness is not a good reason for doing it? Only joking - plenty of other routes 22xs could go to, many of which wouldn't need depots away from the East Midlands to learn how to look after them and where a bi-mode 22x would come in handy.
The Class 22x will not require enormously expensive rebuilds to continue in operation, and as far as I can think of there are no routes that East Midlands Trains or similar operate that will be not be electrified at the same time as the Midland Main Line that would have any significant use for them.
Even Liverpool-Norwich cannot use them due to its extensive SP limits on the eastern half of the route.
There is nowhere else practical to send them than an XC network that will be swimming in diesel IC trains once electrification of the Electric Spine is complete and the Great Western which will have HSTs to replace.
Project THor also appears to be dead, the DfT is still balking at the cost and we have heard nothing since before the electrification projects were announced.
Think you entirely missed my point. When the 180 fleet was being run down by FGW in 2008-9 they replaced them on Cotswold Line services with HSTs but soon realised this was a very expensive way of carrying a lot of air up and down outside the peaks. As a result, the line went backwards from the 2004-8 period of intercity standard trains (HST or 180) on the large majority of services, as back came the Turbos outside the peaks, because they were all that FGW had with a capacity better suited to off-peak loads on the line, even if unsuited to journeys of two hours-plus to/from Worcestershire.
What constitutes InterCity standard? You seem to not want catering of any significant kind, which is about the only thing that sets IC apart these days, and at the same time complain about HSTs being replaced with Turbos that have little else wrong with them. (Apart from perhaps the lack of air conditioning which would be fixed by ordering new flat front DMUs).
So are you saying that Swansea should keep HSTs into 2018, irrespective of wires being at Cardiff by spring 2017? Why would you not run bi-modes on Swansea services if you could while wiring is completed west of Cardiff? And good luck with writing a GWML timetable for that period, given the drastic differences in performance between diesel and electric traction. Even pathing the few West Country trains that go via Bristol will pose a challenge on an otherwise all-IEP railway west of Reading.
There is a simple answer to the pathing issues with all the mixed performance trains... which is to simply cap the performance of the IEP to that of the HST during the transition.
Since the transistion will only be 12-15 months at most, and since we know precisely which services will be transitioning in which order creating transitional timetables if one is really required will not be a major issue.
Delaying the official launch of the all-singing all-dancing new timetable until after the transition is complete would avoid embarrassing PR disasters like the one that delivered the coup-de-grace of the APT project or the U-turn over Operation Princess.
It woudl enable trains to be shaken down gradually with the new timetable only comign in once the entire fleet is in service.
As HST leasing costs will be run down in a big way towards end of life as they will not be DDA compliant and will have little value other than as scrap and museum pieces, extending the fleet's life by about 18 months for a small fraction of them would not cause massive economic issues.
EDIT:
If having the "flexibility" of 2 5-car sets is preferable to a fleet of 9-car sets.
Why not order a fleet of 5-car sets and not bother with 9-car sets at all?
You could push to 2tph to Swansea or somesuch by portion working the Bristol trains rather than 1tph using a 9-car set.
