That assumes there is a second IEP waiting at Oxford to double the formation. If the full IEP service proposals published some time ago (3tph Bristol, 1tph Weston-S.-M., 2tph Wales, 1tph Cheltenham, 1tph Coltswolds and 1tph Westbury) are delivered then the number of diagrams doesn't look sufficient to run any doubled up IEPs, unless the hourly PAD-Oxford fast that doesn't go onto the Cotswolds (which I guestimate as 3 diagrams) is not an IEP service. Even then, three diagrams is going to leave fair number of solitary 5-car IEPs out and about, given there don't seem to be enough 9-car electrics for the Bristols, Swanseas and Cardiffs.
This is one of my gripes with IEP. Without IEP, Weston-Super-Mare and Cheltenham might have a good chance of seeing wires in CP6. With brand new IEPs on these routes they might have to wait until IEPs are life-expired for wires. The logicall thing to do I suppose would be cascade bi-mode IEPs to XC (although 5-car units are probably not long enough there either) but (perhaps unless you confined them to the Plymouth-Edinbrugh core) the IEP's longer coaches would make that an expensive move due to route clearance. What Weston-S.-M. and Cheltenham need is a stop-gap measure for a few years (assuming IC125s can't get a short exemption to allow their unaccessible doors to remain in use until those routes are wired), IEP bi-mode is much more of a long-term solution which could jepordise electrification*.
* Unless you use old engines under the IEPs instead of brand new ones, so you can throw the engines away after just 7 years use. I doubt those brand new 700kw MTU lumps could be ditched after such a short time.
Only 4 longer bi-modes, at the cost of shorter electrics? That doesn't seem to solve anything much.
Maybe they would be the best trains for the Cotswolds line, but they are far from the best trains for anything else. This means that, if they are used, they would be a micro-fleet. I would also contend that 7/8-car bi-mode IEPs are probably better for the Cotswolds than 5-car ones, forget splitting IEPs.
Assuming for the moment that:
- 222s can't be converted to bi-mode and aren't coming to the GWML
- The GW Intercity fleet is going to consist only of IEPs, IC125s and class 180s
- The line to Westbury/Taunton is not electrified west of Newbury.
- The Pembrokeshire/Carmarthen workings are transfered away from Intercity Great Western and operated to Bristol/Cardiff/Portsmouth instead of Paddington using diesel-hauled mrk3s
- A stop-gap measure is used for Weston-S.-M. and Cheltenham until they are wired
I would suggest the following GW IEP fleet:
- At least 36 9-car electric IEP diagrams, without the 'donkey engine' currently planned
- 7 IEP bi-mode diagrams for the Cotswolds line, in at least 7-car formations
- 4 more bi-mode IEP diagrams of the above length for the Paddington - Westbury semi-fast (minus the extensions futher west)
Even if the electrification isn't a rolling programme and stops at what has been confirmed the ratios are all wrong, it should be around 25/26 electric diagrams (including the Oxford fasts) and arround 21 bi-mode diagrams (again, longer than 5-car) rather than DaFT's 18:32 ratio with 9-car bi-modes.
A nice way of doing a stop-gap measure, I think, would be refurbished mrk3s and new electric locos, swapping the loco for a 57/47/67 at Swindon/Bristol until the wires are extended. After then the electric locos could run the whole distance and you wouldn't need the 57s/47s anymore.