"midland mainline stock" meaning the East Midlands Railways Class 222s?
It has been discussed in depth in other threads.
Indeed, including the suggestion of fewer but longer 222's so they don't need to be paired with anything and maybe lengthening some of the 4 coach Voyagers to get a neat universal 5 coach fleet.
It was proposed a long while back (2011 ?) that the 220 & 221 sets (and maybe 222 sets) could have an additional coach with a pantograph to make the sets bi-mode under the code name "Project Thor" that has been discussed several times on the forum. It came to nothing mainly due to politics I guess.
There were some engineering issues, see below.
Wasn't there a load of issues such as:
No high voltage bus line between cars
The assembly line having been long since dismantled
New vehicles would need to meet modern standards
etc?
IIRCa a lot was down to needing to have 2 pantograph coaches added to the 5 coach units, increasing the number of new coaches beyond what was thought to be necessary. In part as it would have meant an increase in seating from 260 to circa 350 on about 1/3 of the fleet, which would have added 30% capacity across the whole fleet.
However it doesn't appear that the suggestion was made to directly replace a coach in the 5 coach sets and then run a smallish fleet of 8 or 9 coach units from the excess coaches.
A 5 coach units would have required 2 new coaches and one excess coach to create a 8 coach units, resulting in a fleet of 10. Whilst it would have required two excess coaches to create a 9 coach units, resulting in a fleet of 7.
However that would have meant that rather than 42 pantograph coaches for the 5 coach fleet it would have been 27 to 30 (giving a total of about 65 rather than 79).
Now bearing in mind that a 8 coach units would have had broadly the same capacity as a 4+5 pairing and a 9 coach units as a 5+5 pairing it would have allowed the retirement of the HST's and still provided a reasonable capacity increase (as there would be no 200 seat 4 coach units as they would all have been 260 seat 5 coach units adding about 30% to the capacity of the units) as well as a few extra sets which were the same capacity as a pair of units before the upgrade allowing a few extra days to run in pairs (at least over the core).
It may not have saved the project, but it could have made enough of a difference to have made it happen. As not only would it have been cheaper (especially as it could have seen the HST's withdrawn) but the total fleet capacity increase would have appeared more reasonable (depending on the option taken forwards somewhere about +10 to +20% rather than +30%) and the operation of the services would have been better (universal 5 coach fleet other than a few 9 coach units which could have allowed a 9 coach units or a pair of 5 coach units to provide the same service without much issue on overall capacity). Even seat reservations could have worked easily, for instance first class is in coach X and Y (2 different end coaches in the pair of units or the end coach and the half coach next to it on the longer units) with once end coach or part of a standard coach not having any reservations. It may have resulted in some odd letter formations (depending on which way around each set in the pair was) but it could have worked.
Chances are we'd still be taking about overcrowding on them now, however from a much better baseline.