The loco would have to run around at Edinburgh regardless so it doesn't matter that much which portion is where.
Doesn't the electric loco off the Highland Sleeper lay over in Edinburgh all day? Surely electric traction is cheaper than diesel and for a journey of 80 minutes with four stops, electric to Perth would be attractive with reduced journey time and better use of electric loco. It has to come off at Edinburgh anyway so tying it on to the Inverness portion is no more complicated than adding a diesel loco. Are there shunters already in Perth at that time of the morning and evening to exchange locos there?
If the route is electrified to Perth wouldn't it be better to avoid Edinburgh and join/divide at Stirling with the Inverness portion continuing to Perth with the electric from London as the front portion.
If the business from Fife is not substantial, would it not be better to drop off the Fort William portion at Stirling and divide Inverness and Aberdeen portions at Perth.
I would like to see the sleeper traffic to Scotland build so that Inverness/Fort William with even a portion for Oban dividing at Crianlarich can continue from Euston but Aberdeen from Kings Cross via ECML with stops at Peterborough, York, Newcastle dropping a portion at York for Leeds, Huddersfield, Manchester and Liverpool and picking up a portion at York from Liverpool for Scotland. Similarly in the reverse direction a portion from Liverpool added to the southbound service at York with a portion dropped off for Liverpool. Both portions to/from Liverpool run as one train to York before dividing for respective north/south services.
Those travelling beyond Edinburgh transfer from day coaches to sleepers at York or day coaches to day coaches at Edinburgh as only the Aberdeen portion travels beyond Edinburgh.
The Lowland sleeper runs as currently from Euston but to Glasgow only with a portion to Paisley, Ayr, Girvan and Stranraer for a passenger only hydrofoil connection to Belfast.
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Which portion of the highland sleeper is behind the 87/90/92 when it arrives in Edinburgh? I can't see the point in getting an extra electric locomotive to take a portion away from Waverley and then have to swap it again when the wires run out. Until wires reach Aberdeen or Inverness I don't see it being at all useful to swap locomotives again, remembering that there would be no difference from the perspective of the carriages themselves (unlike the move to electric for passenger trains, which means being able to run longer EMUs more frequently with less noise and vibration - none of these apply to the sleeper). It's only one train a night in each direction and there's no need or ability to interwork with any other services.
You would not require an extra electric loco. As I have said above, the electric has to come off at Waverley now so just tie it on to the other end of the Inverness portion as far as Perth. The diesel loco that currently operates Inverness-Edinburgh-Inverness would just work Inverness-Perth-Inverness. Simples. I agree an additional traction change would be necessary at Perth if this train continues to divide at Edinburgh.