Remember the environmental factor. The days of cheap short haul airlines may be numbered.
Channel Tunnel services nearly happened (Nightstar) but were strangled at birth by budget airlines.
In the UK I wonder whether it's worth replacing the lowland sleepers with a West Country to Edinburgh/Glasgow service.
How about a South Coast to Wick or Thurso via Reading & Birmingham?
A little story for you about the Channel Tunnel, Sleeping Car trains, and demand modelling:
Once upon a time, in a ministry far far away, there lived a man whose job it was to come up with bright ideas.
One day he thought "I know, that Channel Tunnel project has been given the go-ahead. How about running some sleeping car services to link the outlying parts of my fiefdom with the Continent.
So he set about finding out if it could be done.
He soon found that it was going to be very expensive, and take lots of locomotives and specialist generator cars to work such a train. Such extravagances could only be justified if there was a really heavy demand for it.
He turned to the figures produced by his demand modellers. "Gadzooks" sayeth he, "The folks of Swansea and Plymouth are always making journeys overseas. They'll be ideal for this."
A timetable was drawn up, offering services from Swansea and Plymouth (and elsewhere) through the Tunnel to various points Continental. Coaches were built, a fine selection of knackered old 37s were refurbished, and everybody looked forward to the service starting.
Something was niggling away at the Man from the Ministry. He couldn't quite believe the demand figures, so he went back to the originals and dug a little further. When he looked more closely, he realised the fact that the demand for "Continental" travel from Swansea was actually people travelling from Swansea to Cork. Likewise from Plymouth it was passengers for Brittany and Santander, plus assorted naval personnel off to every point of the compass.
The man in a ministry far far away had an even better idea, which was to pull the plug on the entire project, and flog off the unwanted coaches to Canada.
Thereafter he was very careful when it came to demand modelling, always making sure he knew what question was being asked, and of whom.