I don't think that this is a hard problem, if you look at it the right way: after the fact. It is the end state that is important.

Here are the end states:

Choice 1: You pulled the alarm when you believed there was reasonable cause. An ordinary person would believe the same. ------

Outcome 1: It was only a bag, so you look foolish and maybe have people annoyed that you stopped the train for nothing.

Outcome 2: The person could have damaged their arm seriously. You did a good deed and look good and maybe people are annoyed that you stopped the train.

Choice 2: You do not pull the alarm when you believed there was reasonable cause, as above. ----

Outcome 3: Only a bag so the person is annoyed that you did not and maybe other people.

Outcome 4: The person suffered severe injury. People are annoyed that you did not stop the train. You are clearly morally wrong and have lost part of your humanity.

Given that you reasonably thought the person's hand was in the door, the only ethical and moral action is to stop the train. It makes no difference what actually happened after the fact.

While the other possible outcomes may be unpleasant, the one where you stood by and let a person be hurt is the worst.

This is not an idle speculation. Once I was driving on an inter-state and a person who was seemingly drunk came zooming up, weaving in and out of traffic crossing lane lines. I could have called, highway patrol and did not.

My exit came up and after I took it I heard on the news that there had been an accident on the highway, in my direction a few minutes later. I don't know it was the drunk, but it could have been. No math, no analysis will save you from losing some of your humanity when you could have saved someone from harm and choose not to do so. [formatted for better clarity. Hopefully]