That is not at all how it works. All trains will be rostered to be covered under the base roster.To some extent it must have been decided to roster crews for the earlier trains, probably leaving later ones on overtime etc
The problem comes when you have Driver A route learning, Driver B stood down after a SPAD, Driver C off sick following a minor operation, Driver D moved to another TOC but it just so happens they are all on late shifts. If you only have 1 spare driver on lates you have 3 uncovered diagrams that were all covered in the base plan.
The links are designed so that assuming the depot is at full staffing every diagram is covered every day. Jobs aren't left out of the links to be covered by overtime. And when there are gaps in the links, the uncovered jobs will change each week.
