Interesting replies, thanks. So if the junction issue was sorted out (however unlikely it might be these days), then journey times from TWB would decrease? That would be nice.
Eh it's not exactly that simple.
Capacity is limited by the single track sections. The capacity is perfectly fine for 2tph in each direction. Any speeding up of journey times will also need to confirm to the same infrastructure, and the same single track sections. If you could speed up the junction, you might shave a minute or two off.
Currently, the section from Portobello Junction to Shawfair is a single track section. The junction itself is single lead, 15mph in both directions, which chews through capacity on the ECML. It's not the biggest issue as it's the first junction adjacent to a terminal, meaning that a Tweedbank can just slot behind a fast in either direction, and so timekeeping is pretty rigid. But it's still twice an hour, in each direction.
Realistically the best option is double tracking the whole lot from Portobello Junction through to Shawfair, and associated trackwork, but that's a huge job. Making the junction double lead doesn't achieve anything with the single track section still there, so you'd need to build a second platform at both Brunstane and Newcraighall, and rejig all the junctions on the triangle. If you did all this, you'd maybe gain another five minutes.
Even just replacing the the junction itself with a faster single lead is a huge job, requiring probably a week or two of the ECML being closed, or at least the Tweedbank line closed for the duration.
It's not easy.
The easiest way to speed up journeys is actually going to be electrification.