Here's a slightly radical suggestion. Bear with me...
ScotRail's long-distance fleet needs replacing. In addition, we can be hopeful of electrification slowly being extended northwards. So, procure some 3-car EMUs with a suitably comfortable interior. These EMUs can run in pairs or even threes as dictated by demand, and obviously run under electrification.
Then, organise a fleet of lightweight diesel locomotives with autocouplers and suitable ETS supply to haul these EMUs on unelectrified sections. So, at present, this would look like Glasgow/Edinburgh-Stirling on electric, then stick a locomotive on at Stirling, and continue to Aberdeen/Inverness. (In the case of EDB-ABD, the locomotive would be attached throughout.) As electrification gets extended (eg: to Perth, then Dundee, then... probably no further because the country will have lost the ability to electrify at even an excessive cost), simply move the place where the locomotives attach/detach.
The same principles could be applied to the West Highland line as well, attaching/detaching locomotives at Helensburgh Upper (having extended the wires up the hill).
What does this have to do with the sleeper?
Well, these EMUs would also be procured with a sleeper version (well, several variants, eg: 3 sleepers, 1 seated 1 lounge 1 accessible sleeper, etc etc). You should be able to fit six of these EMUs at Euston (18 cars), since you no longer need a pair of class 92s to also be in the platform. The whole consist runs on electric to the Central Belt. For the Lowland, you simply split at Carstairs then carry on, simple. Highland gets split at Edinburgh as now, then: Aberdeen has a locomotive attached, from the same pool that runs the daytime services; Inverness travels to Stirling on electric, then has a locomotive (or two if necessary) attached, same pool as daytime services; and Fort William, same again, runs on electric to Helensburgh to then hauled thenceforth to the end.
This has several advantages:
- Higher capacity due to additional coaches running from London
- Capacity can be reassigned to different portions depending on demand (albeit in 3-car blocks)
- Reduced operational costs due to no traditional shunting needed (all autocouplers)
- Costs of diesel locomotives shared with daytime services, increasing fleet utilisation
- Mechanical commonality across daytime and night-time fleets (both EMU and locomotive), simplifying and reducing the cost of maintenance
- More chance of a "spare" locomotive being parked nearby to rescue a stranded train (also applies to daytime trains)
What's not to like?
EDIT: the locomotives would be fully controllable from any cab of the EMUs, so could propel