[martinald]

There's passive provision (ie: at not a lot of extra cost - perhaps no real new infrastructure, not entirely sure) to run a 4tph service to Gorebridge + continue half of them to the rest of the line.

I think a 4tph / 2tph service pattern is fine and future capacity could be gained by platform extensions + longer trains.

I'd rather they 'value engineer' it to that level now than build a double track railway at significantly extra cost, and spend the ~£100m (educated guess) on other railway improvement projects for other areas (eg, the quad tracking of the ECML near Musselburgh to allow them a 2tph service - will cost around £100m for the various improvements).