jonmorris wrote:

Certainly calling the police on my mobile would have been a good idea,
rather than using the alarm, although the train would have long since
departed by the time they arrived (Potters Bar to Hatfield at that time
of night is only a few minutes) as it wouldn't have had any cause to
stop and hold anyone onboard.

You could have used the alarm after the train had stopped, and
explained to staff that the police were coming. But, as you say, it's
always easier to think of these things after the event.