Could a Darlington – Newcastle stopping service be viable? I don't know who'd run one though
The problem with any stopping service is getting the paths for it. There did, many moons ago, used to be 1tp2h Newcastle-Darlington-Middlesbrough-Redcar/Saltburn. That provided the majority of the service at Chester-le-Street. However the 75mph DMUs were just getting in the way so it was canned.
The same issue would still apply now, even with the 90mph 158s it'd be tough to fit in due to the (relatively) poor acceleration of the DMUs.
Northern can run that way- indeed, they do run that way in the morning peak before the express services are up to speed.
I'm surprised in today's political climate another train from London has been chosen instead.
It's where most people want to go to.
I can't see it being viable to send over a handful of 195s or 331s, unless Morpeth services were also to switch over.
I think a lot will depend on what happens to the Edinburgh-Newcastle TPE services. Clearly TPE don't want them anymore and it's a waste of an 802. So if those services end up with Northern then they will need an 100mph+ EMU for them.
A two-hourly semi-fast Newcastle-Edinburgh would provide a good service to Northumberland, where most people are travelling to Edinburgh or Newcastle. But I think it'll fall down the cracks- Scotrail will take over the stopper north of Berwick but, understandably, there won't be the appetite in Scotland to take over services south of Berwick.