Hypothetical question, because it very nearly happened to me, only my train
was 5 minutes late, and I just about made it.

If I have queued for 15 minutes without being served due to there being only
one person serving and some guy hogging the queue, and my train is about to
depart, can I board that train there and then and still obtain the
discounted rail fare available with my railcard from the on train ticket
person?

Marcus