When Newcastle Airport were looking into a branch off the ECML from Camperdown to the airport via Gosforth Park I assume four tracking from Newcastle to the junction would have been part of the plan also. So it might be possible to do that. Although I expect the difficulty involved is why that plan changed to connecting to the metro line at Benton instead. Pity because you could have a service which runs fast to Station Road (Forest Hall), West Moor, Camperdown, Dudley and Cramlington every 15 minutes. And 2tph running fast straight to Cramlington, Morpeth and Pegswood.
An additional thought, maybe fast trains should be diverted onto a new line from Killingworth to Alnmouth. And a new line can connect Pegswood and Ashington so local services run on a loop from Newcastle - Cramlington - Morpeth - Ashington - South Newsham - Newcastle (and vice versa) with a connection maintained through Widdrington for freight.
- LNER Newcastle - Alnmouth (1tp2h) - Edinburgh 1tph
- LNER Newcastle - Berwick - Edinburgh 1tph
- XC Newcastle - Alnmouth - Berwick - Dunbar - Edinburgh 1tph
- TPE/SR Newcastle - Alnmouth - Belford - Berwick - Reston - Dunbar - East Linton - Musselburgh - Edinburgh 1tp2h
- LUMO Newcastle - Alnmouth - Edinburgh 5tpd
Local
- Newcastle - Manors - Camperdown - Cramlington - Morpeth - Pegswood - Ashington - Bedlington - Bebside - Newsham - Seaton Delaval - Northumberland Park - Manors - Newcastle (3tph both directions)