The up loop is mostly used for looping freight trains. Someone here may be know better, but won't increasing TPE Liverpool - Newcastle to 2tph result in 1tph at Chester-Le-Street (as opposed to its current 1tp2h)?
North of Newcastle only Manors, Cramlington, Morpeth and Alnmouth get anywhere near a decent service (of around 1tph), although based on relative populations, there aren't that many people living there:
Wikipedia Lists:
Pegswood: c3000
Widdrington: c2700
Acklington: <500
Chathill: Unlisted
Compared to
Chester-Le-Street: c24000.
Morpeth: c14000
Cramlington: c39000
You can see why a greater service is not really required (although, i'd suggest Cramlington and Chester-Le-Street are underserved).