MySQL Database Schema for Train Timetables
I would strongly consider explicitly storing all the days a particular schedule actually runs on. This would give a structure looking something like this:



By doing so, you'll make it much easier to answer questions such as "What are all the trains going to station X on 1 Oct?". It'll also makes "temporary gaps" when trains aren't running (e.g. Christmas day) possible to identify. A one-off train is now simply one with only one entry in SCHEDULE_DAYS.

As the schedule can be different on weekends to weekdays, I think it's better to have separate rows for each day. This allows linking different schedules for every day of the week, should you ever need to do this.