It's still only about 50kg per passenger, so still better than the flight + drive option.Just the leg from Edinburgh to Fort William will easily use 1000 litres of diesel, which will produce 2,600kg of CO2e. Add in some more for the shunting either end and the locos idling, and 3000kg CO2 is in the ‘probable’ range. It could be twice that. Not such a good CO2e per passenger number when there’s not many passengers on the sleeper.
Being really provocative, it would be better for the CO2 emissions for all Fort William sleeper passengers to be turfed off at Glasgow* and driven individually by car to their destination, one per car.
*Yes I know the Fort William portion splits at Edinburgh.