An extra Euston-Scotland means 4tph north of Preston.
How will this work with the current flighting policy?
Currently the fast Glasgow goes first from Preston, then the Birmingham and lastly the TPE, with slightly different stopping patterns.
The Birmingham and TPE go alternately to Glasgow and Edinburgh, giving 2tph to Glasgow and 1tph to Edinburgh.
It would appear Edinburgh needs a bigger capacity increase than Glasgow.
You surely wouldn't want 3tph to Glasgow, all leaving within 20 minutes of each other (leaving the rest for freight and some local Northern services).
Network Rail has previously said it can't manage more than the present 3tph.
There's ETCS coming, of course.