Hey!
I think it would be really great to be able to specify exactly which service_ids are included in a particular timetable, as sometimes simply specifying particular day/date/time intervals just doesn't provide the granularity needed.
My suggested implementation would be to create a new optional file timetable_services.txt in which one can specify timetable_id and service_id. Ideally more than one service_id could be defined for each timetable.
Thanks ahead of time!