The current Sheffield - Carlisle service is very attractively timed for those wanting a day out in the countryside (or just a scenic train service and then a couple of hours in Carlisle), but I don't know if there'd be week-round demand.
I think the main demand is for Leeds-Skipton to Wakefield Barnsley from viewing the figures myself while travelling on the train. Not Carlisle Sheffield i know that doesn't need a link, i think it is a handy way of increasing connectivity is all.
Which stations are currently underserved on the ECML north of Newcastle? Most get a reasonable service, the biggest "problem" is probably the lack of clockface times and the lack of services *between* them (e.g. Alnmouth to Berwick or Morpeth ought to be a reasonable flow, but many Alnmouth services fail to stop at either of the other stations).
The difficulty is that any "local" service is going to be snared up between the current fast ones (which may be even faster if they can now avoid stopping at places like Alnmouth).
The station which I think does worst on that section of line would be Mussleburgh - but then I was a big fan of the GNER pipe-dream for an East Lothian Parkway - am sure there's a decent market there for people who don't fancy driving into Edinburgh (albeit slightly tempered by the opening of Newcraighall).
You have actually explain my reasoning for this service. It would of course have to be a 110mph EMU running the service, but i think with the loop near Morpeth and the Dunbar loop a two hourly service would be possible. Newcastle, Morpeth, Alnmouth, Berwick, Dunbar, A station to act as East Lothian Parkway and Edinburgh.
Interesting question.
I think that a number of lines have seen a big increase in passenger frequency since privatisation, often at the "cost" of running smaller trains (Operation Princess, four coach 158s replaced by three coach 170/185s on FST/TPE services etc).
Whilst there's a (marginal) benefit for passengers, that means a lot fewer paths for freight. Maybe we should introduce a (minimum) limit on train length on some lines to free up freight paths - like five coach 180s on the ECML south of Doncaster being forced to double up?
I have long thought that GC and HT should be forced to split and join.
Castleford - York would make more sense as an extension of the current slow service from Sheffield to Castleford (via Barnsley) or an extension of the current Huddersfield - Wakefield service (extended to Castleford in place of the current slow service from Barnsley previously mentioned), as there may be more "longer distance" passengers attracted to that (i.e. nobody from Leeds is going to sit on a DMU to York that takes an hour to go via Castleford when there's a regular fast service via Garforth). I would have this as well as what i suggested.
