Unless I very much misremember the limiting capacity of WCML between Carlisle and Carstairs is driven much more by freight capacity over Beattock well before any junction capacity issues at Carstairs come into play.
Indeed. Capacity constraints in order of importance on the northern WCML;
(Some of these have caveats, which I shall explain. While I'm well aware many on here here have heard all this before, let's go back through it)
1. Flat junctions at Rutherglen, Newton, Uddingston, Motherwell x2, Wishaw and Law Junction mean regulating any and all traffic north into Glasgow is a nightmare. It's incredibly difficult to get a free run in, and even the via Carstairs ScotRail services have pathing times of between 4 and 19 minutes added through this section. It's a nightmare
2. Haymarket corridor means paths into Edinburgh are fixed. No renegotiating without waiting for your slot at Haymarket. Anything coming from Carstairs usually has a timetabled wait of about five minutes here just to wait for its slot. It's all well and good speeding up the junction, but the trains will end up waiting anyway further along.
3. Paths south of Preston are fixed. Again, all you'd do is be waiting further south.
4. Capacity up Beattock is difficult at the best of times. Timetabling anything other than a container train hauled by electrics struggles to get up in a timely fashion. There was a rule for a very very long time of one train on the hill at once, essentially making it a 6 mile long single block. Thankfully that has gone now, but even so it's getting a bit crowded.
Carstairs isn't really a capacity constraint. Yes, it's slow and arduous on any side of the triangle at the moment, but it's really not a priority other than clearing the junction a bit faster to clean up the timetable. There aren't really any journey time reductions to be gained, so.why spend the money.