Considering the rarity of something like this happening, you are more likely to be killed by a car, than by a random crazy person with a knife. I ride my bike to a train station to commute, and never stand real close to the edge of the platform when the train approaches. Am I afraid of a crazy "train pusher"? Not really, but it has happened, so it is an easy, even though probably unnecessary, precaution. Do I watch my back in all public situations? Not always. Trying to do that borders on paranoia, and is detrimental to my quality of life. I do avoid proximity to obviously crazy people. This includes making eye contact with open carry nuts, who I have encountered on trips to Arizona (Prescott), and Nevada (Pahrump).