How do I stay calm during serious transit incidents and crowds?

You handle the ordinary disruptions, which is most of the job and no small feat. What still overwhelms you is the sharp end: the serious incident, the crowd that surges, the moment when many people need something from you at once. Being overwhelmed there is not a failure of nerve. It is a human response to a genuinely hard situation. In a serious incident, the crowd feeds off your state. If you are visibly rattled, the fear multiplies; if you are steady, it settles, even before you have fixed anything. You cannot control the incident, but your calm is the one input that changes the whole scene. That is a real power, and it is trainable. Decide now, in the quiet, what your first three actions are when something serious hits: breathe, assess, communicate one clear thing. Having a fixed opening move means you are never starting from blank panic. Rehearse it in your head so it is ready. The crowd and the incident will still be hard. You will meet them from a place you prepared, instead of from nothing.