As far as I know, there’s two links, the lowlander sleeper link and the Highlander.
Lowlander is nightshifts and you sign Edinburgh/Glasgow - London Euston. Highlander sign Aberdeen, Inverness and Fort William (Edinburgh TM’s only go to the Fort occasionally to cover Dalmuir TM’s if needed) and is early/late shifts, either working the train to one of those destinations then passing back or vice versa.