I wouldn't have thought so - Newcastle station isn't particularly congested. There may be an issue with platform utilisation (i.e. a through service having to use through platforms instead of the north end bay) but this still seems a poor explanation.
Whilst at first glance using the current timetable Newcastle isn't the most congested, the current layout just doesn't favour services from the Tyne Valley going through onto the ECML northbound, and even less in future when you add in the extra 2tph each way to Ashington, the LNER services replacing the TPE Scotland services (which currently are skeleton timetable only) and the new First East Coast services.
Its not the station itself which is the main issue (but it doesn't help) but more like the junctions to the East. If you head North off platforms 5/7 towards Ashington you are probably blocking the Southbound (Up Main) line for up to 6 minutes as you can't get to the Down Main until after Manors at Argyle St Junction. (based on the time from Train Ready to Start pressed at Newcastle until the time the Northbound train has cleared the Up Main and the route can be set for a southbound service). Add 30 seconds for a service randomly using platform 6.
The fun really starts when a Northbound Northern train is routed from platform 5 to the North with a stop at Manors with it being held there in the Up platform for a late LNER northbound which hasn't even reached Newcastle yet (no loops currently til Chevington as Morpeth is OOU) and a southbound service also needs to stop at Manors. It happened last week on the Chathill flyer which was held for 18 minutes in the end delaying a Morpeth to Carlisle by 7/8 minutes
Platform 3 and 4 is a bit better but still looking at blocking the Up Main for 3 / 4 minutes, you can get to the Down Main before Manors at Dean Street but again from time routes set one way for one train and route set the other is not ideal. Either way if you are using platforms 3 - 8 you are having to cross the Main Line twice (even if you don't stop at Manors and use the slow line, you still need to cross at Heaton then back again at Benton to access the Northumberland Line) so platforms 1 and 2 are the best options for a reliable service not impacting too much on the ECML and platform 2 is probably going to get quite busy.
As mentioned the station layout doesn't help neither, if services from the North / East side of the station (Morpeth / services from High Level Bridge) are more than 3 car then they can't terminate in platform 5 because of where the signal is so have to run to use 7 or run down through 5 to platform 6. No services from the west end (King Edward Bridge) can't terminate in 8 (could only fit a pacer / 153 which are no longer in use), TPE can no longer use bay platforms 9 - 12 (4 car only which also comes into play if XC sub a 5 car on a 4 car Voyager diagram). Northern services can use the West End Bays but occasionally its a nightmare leaving them as you have to cross the fast lines and if a ECML service is late by just a couple of minutes then you are in a queue.
So long story short its probably best to start it as a self contained service and maybe tweak it later however the Metrocentre is no longer the draw it used to be neither so probably low single digits would ride it across town in any case (based on experience of the Morpeth services)
AIUI the intention was to just get to Newcastle and consider through running later. One of the options mentioned in the various planning reports was that it might be run by a micro-franchise rather than Northern (although I think that’s highly unlikely).
But as just posted by
@IanXC through running is no longer flavour of the month. I assume that’s mostly about keeping the Morpeth service isolated from problems off the ECML, because linking the Middlesbrough and Carlisle routes without conflict with the ECML ought to be easily possible given the existing layout. Unless the idea is to put Carlisle trains in the west end bays?
Maybe controversial but Tyne Valley / Durham Coast separation could be to "assist" the punctuality stats as the North East tends to have the higher PPM figures but until we see the unit interworkings across Newcastle it is hard to guess. Being less cynical it may be a case of giving up platforms 7 and 8 for TPE / 5 Car XC terminators instead of the usual platforms 5/6 which means a service from Middlesbrough can terminate then head back from platform 5 and a service from Hexham / Carlisle can do the same from platform 6 instead of having to run a service through from the west through 6 to 5 and back the other end (and vice versa)
