Are things the same for the Edinburgh Festival over weekends?
Yes, even during the festival the last Carlisle train is pre 19:00 on Saturdays and Sundays and weekdays are little better with a last train circa 20:00. Completely useless.
On weeknights at least, last year TPE put on a train to Carlisle during the festival which left Edinburgh at about 22:00. This was very useful to me, so with complete inevitability there's no sign of it being repeated this year. It seemed to be put on in a blaze of no publicity whatsoever. It's been given neither adequate publicity nor a chance to get established so the modest (but by no means zero) loadings I saw are no surprise.
Meanwhile the situation with Newcastle is quite different. Ordinarily the last trains are circa 22:00 weekdays and 21:00 Sat/Sun. During the festival it's 23:30 Sun-Fri inclusive and 22:00 Saturdays. I suppose this is because Newcastle and its hinterland has 8x the population of Carlisle and surrounds but it's still a stark discrepancy.
If you can get to Glasgow and don't mind a slow journey you can get to Carlisle by train at gone 21:00 Mon-Fri and (bizarrely) gone 22:00 on Sundays only, via Dumfries. Hopeless on Saturdays with a last train at about 19:00 though.
All very frustrating, so back to driving into Edinburgh city centre it is for daytrips to this year's festival when coming from the west side of the country.