But can you do this purely by CS services? How would you get from Watford Junction to Euston?
Perhaps I explained the Watford / Euston difference badly. The Central to Queen Street journey doesn't have to go all the way to London as you can change at Watford Junction instead (both southbound Lowland and northbound Highland Sleeper stop there). But the reverse journey wouldn't be possible as the southbound Highland Sleeper doesn't stop at Watford Junction, so you have to go all the way to Euston. The full route would be:
Central - Queen Street:
- Lowland Sleeper (southbound) - Glasgow Central to Watford Junction
- Highland Sleeper (northbound) - Watford Junction to Dalmuir
- Highland Sleeper (southbound) - Dalmuir to Glasgow Queen Street
Queen Street - Central:
- Highland Sleeper (northbound) - Glasgow Queen Street to Dalmuir
- Highland Sleeper (southbound) - Dalmuir to London Euston
- Lowland Sleeper (northbound) - London Euston to Glasgow Central
The Watford Junction changeover
would be possible normally from Queen Street to Central by getting off the southbound Highland Sleeper at Edinburgh then waiting almost an entire day until the next nights Lowland Sleeper leaves Waverley:
Queen Street - Central (shorter version)
:
- Highland Sleeper (northbound) - Glasgow Queen Street to Dalmuir
- Highland Sleeper (southbound) - Dalmuir to Edinburgh Waverley
- Lowland Sleeper (southbound) - Edinburgh Waverley to Watford Junction
- Lowland Sleeper (northbound) - Watford Junction to Glasgow Central
But Edinburgh isn't served by the current Euston - Fort William timetable, so that's not possible. That timetable ends this weekend, so it may change from next week though.