Much as I would dearly love it, the normal turnback is as ECS on a short distance on the non-electrified (normally freight only) Blyth and Tyne
One of the daftest bits of BR-era penny-pinching?
As we've see with other upgrades, you either do a job properly/ fully at the time or you wait a generation or two until you are allowed the funds to fill in gaps that weren't addressed when you made other other improvements (e.g. it took a long time from wiring the WCML to wiring the short section from Stoke to Crewe)
The best chance would be if the often mooted stations in East Lothian & the eastern Borders ever happen - the logical thing then would be to merge the Chathill & Morpeth services into an Edinburgh - Newcastle stopper calling everywhere south of Drem.
Sounds good, but it currently takes seventy minutes to do the forty six miles from Chathill to Newcastle on the existing stopper, (07:10 - 08:20) compared to around half an hour on a non-stop Long Distance High Speed (LDHS) service (passing through Chathill en route from Edinburgh etc).
I accept that an EMU should run
slightly faster than the forty miles per hour average speed of the Chathill - Newcastle DMU but even any Chathill - Newcastle service is going to have to be very carefully timed to get a path that doesn't disrupt the existing LDHS services. Extending that 100mph EMU to stop at Drem/ Dunbar/ Berwick is going to be even more awkward, without adding in the villages of Reston etc (and cramming it into the congested line through Mussleburgh).
I'm sympathetic to the problem of poor local services in Northumberland (e.g. there's only one arrival at Morpeth from Alnmouth in over twelve hours between 06:38 and 16:48 - the 12:57 - that's little use for local travel) - but I don't think that running a local train from Edinburgh to Newcastle with a dozen intermediate stops is going to be feasible any time soon (without building loads more passing loops, insisting that Reston etc are built as four track stations, solving the problem of the flat crossing at Dunbar...).