Operationally the sleepers are complex. Typically the Highlander employs 5 diesel locos:
- 66+73/9 for Fort William
- 66+73/9 for Inverness
- 73/9 for Aberdeen
There are seasonal variations: The Fort William can run with just a single 73/9 when the load is short and the railhead conditions favourable. Sometimes the Inverness is 2x73/9 rather than shed+ED.
These diesel locos operates the evening up service followed by the morning up service.
Meanwhile 2 class 92s are also required for the Highlander: One up and one down. Plus one to draw the ECS between Euston and Wembley, however the Lowlander loco is used.
And then there is the Lowlander operation, which requires 5 92s:
- ECS between Euston and Wembley
- Down Euston - Glasgow
- Up Glasgow - Euston
- ECS between Glasgow and Polmadie - This can be the Highlander loco
- Edinburgh portion (Polmadie - Edinburgh - Carstairs - Edinburgh - Polmadie)
Therefore I believe the typical minimum for 1 night is 6x92, 3x73 and 2x66 = 11