It's still only about 50kg per passenger, so still better than the flight + drive option.
I realise that, and I now realise I got my sums wrong by using the wrong source for aircraft fuel burn
. I’m sending myself straight to detention, for extra homework.
To correct myself:
A typical Luton to Glasgow flight using an A320neo will burn around 2,500 litres of Jet A1. This will create 6157kg CO2e. Typical loading on easyJet flights is well above 90% of the 186 seats, using 90% we arrive at 37kg CO2e per passenger.
The sleeper from Edinburgh to Ft William could burn anywhere between 1,000 and 2,000 litres of diesel, depending on how efficient the MTU engine is (which I don’t know*) how many coaches it is pulling (4 or 6) and how much ‘hotel’ power is drawn (quite a lot, I suspect). This will create between 2665 and 5330 kg of CO2e.
A four coach sleeper formation has beds for 30 people in 15 ‘rooms’, plus 30 in the seats.
A six coach sleeper formation has beds for 66 people in 33 rooms, plus 30 in the seats.
It must be a rare event for all the rooms to have double occupancy, and for all the seats to be occupied given that there is the Edinburgh shuffle with the shared seated car.
On the very best case - all possible beds / seats taken on a 6 car formation, lowest fuel consumption, the figure is 28kg CO2e per passenger. But lowest fuel consumption on a 6 car formation is unlikely.
A more likely ‘best’ scenario is 75% occupancy of 6 car (ie all rooms taken, but half with just one person, 75% seats taken), and fuel consumption of around 1,500 litres (CO2e 4,000kg), CO2e is 55kg per passenger.
Clearly, with lower occupancy, the sleeper CO2e value will be higher still per passenger.
I agree with @Bletchelyite that the most ‘carbon friendly’ way to get to Fort William is electric train to Glasgow (or even Dumbarton / Balloch) and hire an EV from there. The second best would be an EV the whole way. Next up would be to take an electric train to Glasgow (or Dumbarton / Ballcoh) and day train or hire an ICE car (won’t be much in it between these) Then, I’m afraid, it’s likely to be plane and EV, then plane and ICE car before the sleeper.
Whilst the sleeper is many things, for travellers to Fort William it is not the most carbon friendly way, far from it.
* I freely admit that I don’t know the fuel efficiency of the 73. However from what I do know about fuel efficiency of diesel trains it is unlikely that a 73 hauling 300 tonnes+ of train and itself, making frequent stops and three big climbs, and a big hotel load, would manage anything better than half a mile per gallon. And sometimes there’s another loco on the front (like this morning).