> Meanwhile the Borders railway appears to have opened successfully - and is immediately full at commuting times. I hope they left enough space to double track it.

Not for all of it was — there's certainly provision in places, but there's also parts where doubling would be incredibly expensive (the viaducts especially, where the maximum dimensions of the trains it has been built for are larger than it was originally built for, are inherently single-track now).

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).

Several of the completely new bridges were built with single track capacity as well. I suspect the government thought they were being a bit clever forcing Network Rail to shave a few million off the cost this way, but actually just not laying the second set of rails doesn't really save much so actually the savings came in places where they will cause a lot of pain later