From where is the demand for a Penzance-Aberdeen sleeper service likely to come?
Are there that many Cornish oil rig workers?
At what market would it be aimed or is it just whimsy?
Passenger journeys from people joining the rail network at Penzance and leaving it at Aberdeen will be minimal. But then passenger journeys from joining the network at Cardiff Central and leaving it at Portsmouth Harbour are also minimal.
Look to staycation and leisure traffic returning from Newquay and Falmouth, Torbay and Exmouth (and Butlins at Minehead by bus) and Weston-super-mare, Barry and Severn Beach returning home to Glasgow, Edinburgh, Aberdeen, Dundee, Inverness ... Motherwell, Paisley, Livingston, East Kilbride, Perth, Dumbarton. Look at the leisure and staycation traffic from Truro, Plymouth and Exeter, Bristol and Cheltenham Spa ... Bath, Cardiff, Newport, Gloucester and Worcester, headed for a week or weekend away Doon the Watter, in the Highlands, out to the West Coast at Oban, Mallaig or Kyle of Lochalsh, city centres Stirling, Glasgow, Edinburgh, golfing at Turnberry, St Andrews, Gleneagles, North Berwick, Barry Links or Carnoustie. Coach tours with train transit for the long distance stuff. Cruises in to Avonmouth or out from Rosyth.
Look to business traffic where cost, speed and convenience has drive people off trains onto flights from Exeter, Bristol and Cardiff airports to Glasgow, Edinburgh and Aberdeen, often followed by a night in a hotel prior to a meeting. There is traffic to regained as we move to trains being more acceptable / greener, when pricing models get changed, and when space and facilities mean that travel time is spent not just getting a good nights sleep but a good working and relaxing connected setup too.
Remember we're looking at not one but two services a day - every 12 hours not every 24 - and much will be outbound overnight and back during the day, or vice versa. We're looking at a marketed / promoted service, and not something that's accidentally almost there for historic reasons and arranged largely for the medium rather than long distance traffic. We're looking at a root and branch service - a Christmas tree is not just about the fairy on the top, but also about the pretty lights and chocolates hanging from the branches, the baubles and trinkets, the tinsel and branches and the presents around its base.
Yes, whimsy - I have no figures for you. But I suspect some of our other posters might tune a wild seed into a more cultivated product. I'm already noting a potential change of destination of the main train ... looking at Stirling with numerous platforms where the service could usefully start / end, sitting so people can join / leave in really good time and with direct service onward / inward from so many of the key final destinations and starter points.