What if it results in another high speed train rear-ending yours, and dozens of deaths?

The reason you think about things ahead of time is so you don't make an instinctual or socially-pressured decision in the moment which may be bad.

In this specific case, it's whether you even have a brake available for passengers to use, or you decide that having a brake there would usually make the situation worse. Passengers are ignorant of most of the consequences, they can only react to what they see in front of them. The reason you put a brake there is because the driver would likely brake if they could see what was going on, but they can't have and process all the information about everything in the train at all times.

i.e. "If you are standing next to the brake and someone says 'hit the brake his arm is stuck'," and there's a brake for you to use, somebody has already made the decision that your ignorant decision will likely be a good one.

Anyway, somebody's arm getting torn off, a scene spread over a quarter-mile of train tunnel is certainly going to delay trains a lot more than a quick stop, and leave a bunch of people in the train traumatized, with large dry-cleaning bills. There's a way to make this into an interesting problem by changing a bunch of stuff, but it actually isn't. Not pulling the brake isn't going to avoid anything if somebody's arm is stuck in the door. The reason they put the brake there is because they've already calculated this.

Trains do not simply slam into the back of other trains. The recent incident in Spain occurred because the system that would have prevented that was broken, so they turned it off (as it would not have allowed the train to go while it was broken). This reminded the whole world of train operators why they Do Not Do That Ever Ever Ever.

Have you been on a train station and heard an announcement that a train is delayed due to a signaling fault? That is because they are not allowed to ignore a signal even if they know they are broken. The signal must be repaired, no exceptions. When you create an exception you get a train crash, like in Spain, so the rule is No Exceptions Ever.