Apparently he was unable to re-open the doors for the late running passenger because if he did, "the information would be recorded on the black box and I'd have to explain why I'd opened the doors and made the train late".
I didn't know that trains have black boxes as well.

Shame it's not really their fault but that of their management / Network Rail when things go wrong.
Small comfort for the lady left on the platform but I do understand poor Collyn's dilemma. Over the last year and a half, this train has taken longer and longer to reach destination.
Therefore any departure from the time timetable (aka List of Suggestions) is definitely going to bring all kinds of grief down on Collyn.
I imagine that FGW must have this list Gestapo like department who diligently download all the data from the trains and look for any problems and mistakes. This information must then be passed up the line to the enforcement staff who probably take Collyn aside at the next opportunity and beat him with sticks..
That's why he has a limp.