But would those 40 miles or so be electrified just by themselves, just for the use of trains from London, or would they be part of some hypothetical larger scheme covering the whole area? If it was just purely for the benefit of trains from London, and DMUs still had to be used on the local services, would that possibly ever make any kind of economic sense?
I'm not advocating the electrification of every branch off a main line, you understand, but logical extensions of the trunk route that really should be included as part of the main scheme: I'm thinking here of the current electrification of the GWML not including Cardiff to Swansea, or a hypothetical Transpennine North electrification scheme just covering the "core" between Manchester, Leeds and York but not including the likes of Hull and Middlesborough.
Of course ideally, electrifying such sections would also allow local services to transfer over to electric operation, or the Intercity/regional services that are the focal point of the electrification scheme would form the dominant traffic type over the route. Electfiying Leeds to Hull would fall into the former category, but the benefits of electrifying Cardiff to Swansea wouldn't acheive this aim as many of the local services west of Cardiff continue on beyond Swansea.
There might be other practical solutions other than bi-mode for branches that would not form part of the electrification scheme for the main line they are linked to: If the route to Plymouth/Penzance was to be electrified, assuming here that this would include the Berks and Hants, then in the case of the Newquay branch, if it was not o be electrified it would probably lose its' direct London service, but this could be compensated for by increasing the number of Crosscountry services (IMO the Crosscountry network will not be completely electrified for many decades, if ever) to Newquay. This would allow access to and from a greater range of destinations across the country by direct train, and also provide interchange opportunities with both the Plymouth-London services and with the South Wales-London services further up the line at Bristol.
And talking of the power generation being done by a single source, while that may be so, what would this power source be? Since successive governments have run down Nuclear power to appease public opinion, wouldn't that have to predominantly come from fossil fules; and so wouldn't the increase in emissions from them (and the amount of coal or gas that would have to be imported) quite possibly at least be the same as, if not considerably outweigh the savings from not using Diesel trains?
The method of power generation is very much down to the political policy at the time. With Intercity trains having a typical lifespan of 30-40 years, there's likely to be quite a wide variance in the energy policies across their lifetime. An electric train can benefit from any moves towards more sustainable policies on energy generation, whereas a diesel train will always be diesel throughout its' life (Barring possibilities like running them on vegetable oil, etc, which would still require some modification to be carried out).
