Wow, that's a helluva how-do-you-do, isn't it?
This is what's called a situational question, it is a technique used in a lot of industries. Your interviewer doesn't just want to hear your answer, they also want to hear how you arrived at your answer. These questions are generally not about emotion, they want to see how you would think a situation through. These questions are common in many professions. When asked for a question like this, think of the acronymn STAR -- Situation, Task, Action, Result. Your interviewer will give you the Situation, its up to you to come up with the Task, Action, and Result.
Situation: A person is on the tracks.
Task -- What's the task here? Addressing person on the tracks while minimizing injury to your passengers? Anything else?
Action -- What action do you take? Would you slow the train down at all? If you wouldn't stop, why? (Because 150 tons doesn't stop so easily?) Can you take any other measures such as sounding a horn or flashing a light?
Result -- What was the result of the action you took? Perhaps the person was someone intoxicated and the sounds and lights scared them off? If hitting the person on the tracks is unavoidable (such as a person intent on suicide), what then?
I don't know what exactly they are looking for, just throwing ideas you can work through and think about when constructing your own answer.
If you have an idea of other types of situational questions that are asked in the interview, work out your answer in the STAR format ahead of time. Practice them out loud, and rehearse them until you sound confident to yourself. This will take an amazing amount of stress off you when it comes time for the spotlight of the interview.
As far as appearing emotionally detached, I wouldn't worry too much about that. In an emergency situation, one's body is going to be in an adrenaline dump. The ability to compartmentalize your emotions and follow a logical procedure is absolutely essential to working through that situation. If you practice your answer until you sound confident, you won't sound emotionally detached, you'll sound like you are taking the questions seriously.
If they throw questions out there that you didn't expect (which may also happen), keep your answer in the STAR format. Use the actual words if you have to "In the situation of X, my task would be..." Having this format as a framework will keep you focused in the heat of the moment.
Does that help at all?