If they're late, they miss their path and sometimes end up being stuffed whilst they await the next suitable margin (note the reference to trains approaching Bedford in the run up to the morning peak, presumably on the Up rather than the Down as stated - if they miss their path, they can end up standing north of Bedford for two or three hours until it calms down again), they can miss their slot in a busy terminal or drivers can start going over their hours.I never thought that freight trains need to run to time. RTT shows that most freights are either early or late but never run close to time like a passenger working.
If they're early (so that they're in front of a train that they should have followed), there's a risk that they'll arrive at a key junction with no suitable margin to go anywhere, or end up delaying that (now) following train in a different way somehow. Early running is generally discouraged, unless there's a good reason (such as to avoid a conflict that might otherwise arise, say due to a late running train) or everyone's absolutely certain that the train can at least get to a location (without delaying anything else) where it can be regulated back to time if necessary. Some can still run quite early in their booked path though, if the previous train was quite some time earlier, and others can gain quite a bit of time on their schedule so end up running a bit early.
All good fun!
