Sleeper trains are actually very very eco friendly, traditionally, they'd literally be a couple of carriages stuck on the back of mail train which would be running anyway, giving you an extremely tiny footprint.
But running predominantly electric trains at night when power is all generated from renewable sources at night is very good for the environment. A bus is nowhere near as good for providing similar service or for a service even remotely as good in terms of GHG emissions.
Well if it's all renewable at night then presumably the nuclear and fossil stations are completely shut down ?
How can the sleeper as being 'eco friendly' compared with other forms of rail travel. It's a low density service. Even if it was electrically powered all the way arguably you'd be more efficient sticking the passengers on ordinary seated services with higher densities. And worse - in the real world what we have with the highland services are paired diesel locomotives hauling a few coaches with low density occupation over the northern parts of the route. How do they compare with a well utilised 156 or 158 set?