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The problem is that intervening could escalate the situation further, but as soon as the BG (Bad Girl) started "waving" the gun around, I think everyone on the train was in a life and death situation. I would have done what I could to quickly incapacitate her if possible. A hard punch to the back of the head might have been effective, depending on the individuals involved. Tough situation where you could potentially have a struggle with an armed and unstable person that is waving a gun around, and very easy to have it go off while you are trying to deal with the situation unless you handle it with immediate and incapacitating force.

Now it would have been really interesting if the hero had been carrying. In that case, his best course of action might be to take a safe head shot, assuming one is available.