This goes for all lines, not just depot lines. Someone also reported this under Lines Timetable in Suggestions. It connects at the schedule level I believe. For example, I incorporated three trains/shifts that operate line A into a schedule for line B, because these shifts are really split parts of trains on line B. Line B has 20 shifts regular, line A has 12 other shifts of its own trains.

The result, the line B timetable has 23 trains shown, including 3 trains that never operate line B but are in the schedule together with line B trains. This is actually helpful because I can see how these trains align with the timetable for the ones they "split out" of. But the issue is that for line A, the timetable includes 12 own shifts, 3 of these split shifts, and 20 shifts that never run line A but are in the same schedule with the 3 trains that do. So line A timetable is 35 trains and not good.

Understandable on some level but annoying on another. It makes it more work when determining how many tracks a depot actually needs.

Agree. For larger lines it also makes the timetable unusable because it only fits 31 trains.

Maybe being able to toggle whether the timetable view of a line shows all trains within a schedule or only those with the relevant order would be nice. Similar to how you can toggle whether a station shows trains passing through or just those that stop