Not exactly.
Thameslink can get you to St Pancras. Once there, you can go to Kent, Sheffield, the East Midlands, and even Europe. You can also walk to Euston or King's Cross, for the North East and North West, Liverpool, Manchester, Birmingham or Scotland.
Just taking the tube opens up a lot more places too.
So why waste a path on a service that could easily be done if you changed at London? What's the point of an indirect service that takes a longer time than it would to change at London?
By that logic, why have a cross country network at all ?
You could probably get away with having one intensively operated route from York to Exeter with everyone else changing to London services and it would probably be as quick with only one or two changes.
Except that lots of people prefer not to chance, which is the whole point of this thread in the first place. It's hard to think of any other hubs that would be suitable for a long distance ctoss country service, except perhaps a North East/South West service via the East, rather than the West Midlands.
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Where do the units you use for the extension in the off-peak come from? If you look at the present extension of XC to Guildford as an example, it only works because it is done at either end of the day, very early and very late, and the unit runs ECS from or back to the depot. In the middle of the day you would have to feed extra units from somewhere into the normal peak clock face timetable OR you'd have to reduce frequency on a core route to provide the stock.
I just can't see XC's fleet size being arranged so that there are sufficient spare units to run mid-day extensions.
The OP merely asks "where should XC serve" I'm merely saying where I think XC should serve in the event that adequate stock becomes available.
