You've obviously not come across the NYMR, then! You rarely know which loco you have for the day until you sign on. Loco duties are generally only posted up the day before and even these might change by the evening when the locos come back on shed. Like the NYMR, Swanage runs a 7 day/week service for much of the year and planning ahead is Utopia. Locos come back on shed with broken springs, blowing glands, etc. and with 7 day running there is no midweek down time to carry out the required minor repairs and have a reasonable certainty that the loco will be available for its next booked turn at the weekend.