What I cannot understand when cancellations are announced the day before is why there STILL has to be short forms.
One way this can happen is that you have an in service failure, with no spare stock to replace it anywhere near the failure. Say, one of a pair of units has something that means it needs to come out of service, half way through the day at Sheffield, and the nearest spare unit is Nottingham depot. And there’s no spare driver to get the train from the depot to meet it at Nottingham.