Utilitarian choice is to flip the switch. As much as I like to think I would pull that switch, I am not a Vulcan and would probably let it go on. Inaction vs action.
I would probably justify my regret by saying there was nothing that I could have done and it's the train companies fault.
I also think that when it came down to the line, more people who said they would switch the tracks would be unable to do it either.