Tricky one this: I understand exactly what grahame means, but I think the practicalities would rule it out. I tend to view those "expected" arrival times as exactly what they are, which is to say the scheduled departure time of the train with its
current delay factored in, rather than a prediction of what time they may actually arrive.
I suspect that there are so many variables that it wouldn't be possible: it's difficult to predict just how long a train that's out of its booked path might get delayed, depending on what it's following, for example.
Southern Stag makes a very good point: I would say that it's generally better to give a slightly optimistic delay forecast so that people don't go wandering off then return to find the train has made up time and they missed it.
Quite apart from the fact that the 1855, showing on time, is already nearly 2 minutes late.
Different
TOCs▸ specify different thresholds: departure boards at
SWT▸ stations will show delays of 1 minute and above. However from observation,
FGW▸ -managed departure boards will show a train as "on time" until it's delayed for 3 minutes or more.