Yes it could be possible to run full length trains, but that is likely to require more coaches so that you always have a spare to cover for a unit or of service. With 5+5 working you can have 2 X 5 coach units spare which could sub for a 9 coach unit, could swap with a 5 coach unit (either a single unit or one of a pair), it could even allow a service to be run short firm with a 5 coach unit if one of the spare units had already been used. If you had all long units you would either only have one unit spare and in that last case you would need to cancel the second service or you would need two long units say as spares.
You would probably also find that with long units some locations would have a less frequent service.
Yes 10 coach units would be useful, and given the size of the fleet of 80x's it's likely that the could be a follow on order as part of the next franchise. Which could also extend some of the 5 coach versions to 9 coaches as well.
You do however have to be aware of two things. Firstly is taking then long enough to get the 5 coach units into service, if you halved the rate at which units were available then that does down training of staff and the ability to get then into passenger service. This would make matters worse now, even if once they were delivered on some services things would be better. Even if you delivered some 7 coach units for training first they would still take longer to deliver than a 5 coach unit and would be of no use during the peaks. This would then mean that the total delivery time could have resulted in it running beyond 2020 (which is important given that trains have to be compliant with DDA rules by then).
Secondly, if you run 7 coach and 10 coach units and the only spare units you have in the peak are 7 coach units and a 10 coach unit fails you are stuck with running a 7 coach unit. By making them 5 if you have two of them spare you have a significantly smaller difference in capacity when you need to use them (even when compared to a 10 coach unit). Yes if there's just one is more of a problem, but given the numbers involved chances are you could get turnaround a wrong diagram to run it late as a peak departure to then use the 5 coach on a slightly quieter service a little later.
The DfT didn't allow them to have so the coaches they wanted.
They did get it wrong
Likewise they got it wrong
DfT error
DfT error
... And a lot of then would be down to DfT errors. Especially franchises let as no growth and where there's too many short units.
Given how much that GWR promised as part of its franchise extension and how much they have been crippled by NR taking longer to wire the GWML I would suggest that they aren't doing too badly. Should they have said that ScotRail shouldn't have got their HST's quite so fast, maybe but other than that there's not a lot more that they could do.
However then we would just be having the discussion about why ScotRail was doing badly due to matters beyond their control.
Decisions made 20 years ago, or for that matter 6 years ago, have an impact on the railways now. For instance if Northern had been allowed more units target than a zero growth franchise there would likely be more DMU's to go around now. Likewise if TPE had been allowed their 4 coach 158's and or Southern had been told they could buy new DMU's rather than being left with getting cast offs from the North then there would have been more DMU's to go around.
Likewise if the 2012 franchise for ICWC had gone well there would likely be more 221's for XC to use.
Given the DfT ordered the first lot of IETs I am surprised they are not to blame? I mean almost everything the franchises do seems to be the fault of the DfT. Hitachi are even to blame for the training issues.
Of course with 2x5 you have bought the 10 cars already, you have simply doubled the kitchens, cabs and crew and lost a full carriage of seating.
I am not sure I really follow the point about training and unit length. You should order short trains because they are quicker to build meaning the training starts sooner? Really?
A 1x7 in the event of failure has far more seating than 1x5 but as I have said that is not a choice other TOC using non divisible formations make.
