Jonathan Morris:
The doors then opened and the (female)
driver said 'We are aware that the alarm has been pulled. Would the
person that did so please make themselves known to a member of
staff'.
...
If it was someone being attacked, why let them run off the train?

Richard J.:
So that they could escape from their attacker? Or perhaps you meant why
let the attacker run off the train?

For what it's worth, on the Toronto subway system when the passenger
assistance alarm is operated, at the next station the doors open on all
the *other* cars, and a crew member comes to the car in question to find
out what's happened. Or so I understand; the only time I ever actually
used the alarm, we were already at a station with the doors open. (We
still have two-person crews here, so all points on the train are within
150 feet of a crew member. I expect they would usually walk through
the train unless it was very crowded.)

Bearing in mind that there is no way for the driver to know whether
the alarm was pulled for a medical emergency or an attack in progress
or some other reason, what would your solution be?

Keep the doors closed at first, but be ready to open them quickly. Of
course, this might be the wrong thing if the emergency was a firebomb.
--
Mark Brader, Toronto | "Ah, determinism (likewise, forgetfulness) reigns."
| --Steve Summit

My text in this article is in the public domain.