Even the best-laid plans can result in the train leaving the track.  Avoiding the train wreck in the first place is one objective of planning but the unexpected and undesired can and will occur (a little variation on Murphy’s law)

The way you respond to a derailment will have a significant influence on the impact.  Panic, act defensive, ignore or any similar behaviours are not likely to resolve the crash and may in fact exacerbate matters. 

Take Time to Understand

The first step is to understand what has happened.  This could be done quickly within the moment or it may take a little longer depending on the magnitude of the event, however,  time is of the essence

Although it is important to react quickly it should not be precipitously, an inappropriate reaction is as bad as a slow one (perhaps worse).  Recognizing a crash has occurred or is eminent is key to being able to respond appropriately.

Relax

Emotions such as anger, frustration and fear will contribute little assistance as you work to determine the best course of action.  In fact,  these emotions are probably more likely to lead to escalation. 

Of course this advise is more easily given than practiced.  While a quick response is usually needed to start the process of clean up and getting the train back on the track I find a thoughtful pause is almost imperative. 

That is, before reacting, which will typically include some emotional motivations, think through how bad the damage appears or potential to become, in some cases the need for an immediate response is not a urgent as you might initially think. 

Remember, derailments happen, the fact they occur is not as nearly important as how you react to them.