I'd been looking in Network Rail IEP stuff though, where it isn't mentioned. That seems to be considering capacity for EGIP (up to) 8 car units?
Well it's essentially a zero sum game. Anything which creates capacity for some services also creates overall capacity for others.
At the moment passenger services entering Waverley from the east consist of something like:
- Scotrail - North Berwick (18 out of 22 terminate Edinburgh on weekdays)
- Scotrail - Dunbar (3 out of 4 terminate Edinburgh on weekdays)
- Scotrail - Borders (29 out of 33 terminate Edinburgh on weekdays)
- Cross Country (5 out of 15 terminate Edinburgh on weekdays)
- East Coast (17 out of 23 terminate Edinburgh on weekdays)
These services either use the short east facing platforms 3 and 4 or they occupy a through platform.
As capacity is most constrained in the western throat then services from the east either have to link to onward westbound services or reverse in the platform which uses more platform capacity.
If you build addditional east facing platforms then this frees up capacity in the through platforms. At 8 coaches long the new platofrms 5 and 6 will be able to take all the Scotrail services and most of the Cross Country services.
It doesn't really matter that IEPs won't usually use the new platforms as they will still benefit from the overall capacity increase.
So overall of 97 services coming into Edinburgh from the east today 25 are through services while 72 terminate. Of the terminating services 55 (76%) will be able to use the new platforms if required while 17 (24%) will still need to use through platforms.
Ultimately of course this capacity enables more services from the busier westbound throat to be run through Waverley to turnback east of the station and increase the capacity of the western throat.
This is why the scheme falls under EGIP ultimately as the western throat capacity issues are more problematic than the eastern ones.