Situational awareness is kinda key...

So according to this article from Fox, a man walked on to a commuter train in San Fran, literally waved a gun around for a few minutes before randomly shooting a student, and no one noticed until shots rang out.  As much as I would like to rant about that for a while, I will instead refer you to this discussion of Cooper's color codes.  Please read and apply, and keep your head on a swivel.  Even if you aren't carrying, it could give you a few seconds to ID a threat, take action to avoid, and possibly save your life.