So you'd have diesels running all that way under the wires? Waste of fuel?
On what route? In my plan diesels would not be running under the wires (except between Newbury and Paddington, which will happen anyway) for very long, probablly no more than 2 years (does anyone have expected timescales for GWML electrification?). As I said, as soon as the electricfication reaches the terminal point of each service that service gets replaced with an electric train. If you have bi-mode on Swansea and Cheltenham Spa you are dragging around all those diesel engines and all that fuel under the wires for 30 odd years. Waste of electricity?
The services beyond Oxford are less clear cut, I'm having doubts that it can be done without some sort of compromise. Perhaps 220s from XC, or some 180s, could be obtained and fitted with pantograph cars (better than building new bi-modes, as the life-expiry date of the stock, and therefore the next opertunity for electrification, is brought forward 10 years.)
Seeing as this is the HS2 topic, I'll also say that, if the money going into HS2 could be made available for GWML electrification instead the routes beyond Oxford and Newbury could be wired too, and those services would no longer need to be diesels under the wires. Newbury to Penzance (very roughly) is 250miles and Oxford to Hereford about 90 (60 if you stop at Worcester). At the £1.4million per mile rate quoted for Cardiff to Swansea 350 miles comes to £490m, I'll round that up to £500m for good measure, but it's still under the 700million paperwork for HS2. Granted, the rolling stock will mean that overall it'll cost more than just the paperwork for HS2, but it'll be much better for the enviroment, especially if you use tilting trains to speed up the route to Plymouth.
As discussed by w1bbl3 these routes aren't getting IEP (as things stand)
I'm hearing contradictions on here regarding this, some, like you, say all those routes will stay as IC125s (good), others say bi-modes will go as far as Taunton or Exeter (bad). Surely the better fuel ecconomy of an IC125 on the over 115 miles from Exeter to Newbury outweighs a bi-mode's ability to use electricity (very inificently thanks to the extra weight) on the 52 miles from Newbury to Paddington (only 35miles Reading - PAD if you can only change from diesel to electric at a station stop).
We have some 37s, as you mentioned. Are you suggesting we have engines from 1960 hauling intercity trains in 2020? I know we are all attached to things like Mk3s and old diesels, but we can't use these as the basis for future services...
The new IEP electrics would run as far as the wires before the old locomotives get coupled on. It isn't like I expect the old locomotives to be able to run 125mph flagship express services, just get the shiny new electric trains the last little bit of the way. The southern region used to split an EMU from unpowered coaches and attach a diesel locomotive on an hourly basis when they partly electrified a line.
What is going to run the ECML services (if you take the 225s)? And see my point above about using 1960s engines to haul such services away from the wires...
IEP electrics of course. You could send the IEP electrics direct to the GWML though but this way gives East Coast a uniform fleet and saves the Pembroke Dock service (which I'm told isn't possible with stock with 26m coaches). You could try and find some spare 67s for most routes, but as far as I know the only diesels that can have TDM are class 47s, 57s and 43 power cars, so 47s or 57s are what's needed to save the Pembroke Dock service, and they might as well be used on the Carmarthen services.