The principle I have always worked to is simply that if a line has enough traffic to justify an express through service, it has enough traffic to justify the wires. If it doesn't, then it doesn't!
You then end up with two options. Either a diesel shuttle past the wires, or attach a diesel loco the front of the EMU. The fact that performance will be lower is negligible as by virtue of the fact it's not wired, there isn't enough of a business case for better services. If usage picks up and the wires get extended, then simple - design these locos as mixed traffic locos and they can either be found a home hauling freight (as I suspect a lot of freight-only lines will not be able to make a business case for wire for quite some time), or sell them overseas. Plus, you'll still want to keep Thunderbird locos around. Having a lot more around would probably be a boon when "incidents" occur.
Makes much more sense than dragging diesel engines around under the wires, day in, day out, that will be mostly pointless except at the extremes of their routes. Bi-mode is like having motorail coaches stuck on the end for passengers to bring their cars just in case the train breaks down.
Some principle, since it ignores all sorts of complicating factors relevant to the cost of a wiring project and basically dismisses any prospect of improving a route's fortunes. Never mind the doctrinaire, out-of-hand dismissal of the bi-mode as having anything to offer. Clearly SNCF, for one, would beg to differ on that score.
In the case of both the Cotswold Line and Swindon-Gloucester/Cheltenham, the FGW services account for a very small part of the train services operating at the outer ends of the routes. No-one in their right mind would go to the trouble and expense of putting up wires at places with track layouts as complicated as Worcester and Gloucester for just a couple of trains an hour.
When other wiring projects, such as West Midlands extensions and XC happen, which will involve the majority of services at those two locations going electric, and allow for others, such as South Wales-West Midlands freight to switch as well, then the economics of wiring what are basically simple plain line routes, with the odd point here and there, from Oxford to Worcester and Swindon to Standish, are transformed.
Is usage going to pick up if you provide a second-rate service, operated by mixed-traffic locos with worse performance than the diesel units currently in use? I doubt it. Locos that would not be ideal for either fast passenger trains, nor for the heavy freight trains that dominate that business these days.
Even now, in the case of the Cotswold Line, there are people from Worcestershire, in particular, who drive all the way to the Chiltern Line, or use Virgin via Birmingham, because of the supposed slow journeys that FGW offers, but if I take what you say at face value then it's absolutely fine to have all these cars out on the M5/M42/M40 for years to come, rather than improving the GW service so these people get on a train down the road from where they live.
These carriages with the Diesel engines underneath and the high floors will have high floors for the length of their lives so when you have the wires up and remove the engines you will still have the high floors.
Can you do anything apart from moan, moan, moan? The average height of a 21-year-old British man is 5ft 10in, so I think we might just squeeze into IEPs, whatever the floor height, for a while yet.
