Hence why I'd have it connecting south of Doncaster, splitting in the platforms to serve the Branches from Doncaster.
For the inital network running into Leeds at full size gauge and a connection to Doncaster for Classic Compatible units.
Considering that CC units will not be able to run at double length in most ECML stations, 400m long services could join up at Doncaster and run onwards to York (2tph) and the other portion onto Leeds via Wakefeild 1tph or Hull, or Harrogate via York (Splitting at York), or Cleethorpes (Loco hauled), etc.
400m long platforms wouldn't be too difficult to drop into Doncaster, neither would EU Gauge platforms for full size units to fit, so the line may continue from Doncaster to York and onwards to Newcastle as full size units. Splitting still being facilitated within Doncaster station.
Example service pattern being...
2tph Euston - (Birmingham Intl) - (E Mids) - Sheffeild - Leeds (Captive) (Possible extentions using CC Stock)
2tph Euston - (Birmingham Intl) - (E Mids) - Sheffeild - Doncaster (SPLIT) (All operated by CC Stock)
---- 2tph York - (Darlington) - (Durham) - Newcastle 1tph extended to Edinburgh
---- 1tph Bradford / Skipton / Harrogate via Leeds & Wakefeild
---- 1tp2h Hull Paragon Interchange or Harrogate via York
1tph Edinburgh - Newcastle - (Durham) - (Darlington) - York - Doncaster - (Sheffeild) - (E Mids) - Birmingham CZS - (Bristol, if available) CC Stock
1tph Leeds - Sheffeild - E Mids - Birmingham CZS - (Reading, if available) CC Stock
With of course...
3tph Euston - Birmingham CZS (Captive)
3tph Euston - Manchester (CC Stock, but into the dedicated HS2 terminus with 430m platforms)
The following would be 200m long at terminus, and therefore could be joined at Crewe...
(Not that question marks indicate fantasy services)
1tph Euston - Liverpool via Runcorn
1tph Euston - Glasgow
1tph Euston - Liverpool via St Helens
1tph Euston - Blackpool
1tph Euston - Chester
1tph Euston - Manchester Victoria - Huddersfeild - Brighouse - Bradford?
And that leaves paths available for international services:
1tph Manchester Mayfeild HS2 -
1tph Leeds City / HS2 or Newcastle Central - York - Sheffeild - E Mids
Join at: Birmingham (CZS / Intl) - OOC - Stratford Intl - Somwhere through the tunnel
And of course the good sense of:
1tph from Manchester & Birmingham to Gatwick via Heathrow
1tph from Newcastle and Leeds to Gatwick via Heathrow
Thats a nice comfortable 16tph... although some four track would mean many more services would be able to run...