So having been in Edinburgh a fair bit in the past week, I'm a bit confused as to what the plan is for 12/13.
This is what's roughly planned for the long term. The current single-lead arrangement for 5 and 6 is only temporary. 3 and 4 look like they're going to be rationalised to leave one big servicing block. Annoyingly the station may need to be renumbered again if 18 is made into a through platform, which makes these discussions a little difficult.
This is the view from the bridge:
Where's any joined up 13/6 going to go there? There's only a narrow gap between the track of 12 and the southern ramp, certainly not space for a track and platform. Or is there space if you demolish the ticket office and all the retail space to run 13 the other side of the ramp without fouling access to 14?