Stupid question maybe, but what exactly is a flag stop? If you want to get off at one, I guess you inform the conductor suficiently well in advance and he or she informs the engineer and / or scheduler. But if you want to get on, what happens. Do you need to wave to stop the train, a bit like a bus? Or does the booking system automatically alert the train staff that somebody has booked a ticket and that there's somebody waiting to get on? Or is there a phone number you need to call? Or what?