The train doesn't leave the tracks unless it has a problem and then it would derail so that means the person was a trespasser either intentionally or just got too close to the tracks without realizing it. Engineers will sound their horn when they see someone too close for safety but it can take up to a mile for a freight train or a half mile for a passenger train to stop.

In any case, it is the pedestrian who is responsible unless they are at a crossing with signals that did not activate.

It is sad for the victim and even sadder for the engineer and anyone else in the engine's cab who would have seen someone be struck but be unable to do anything about it. Many engineers end up with short, and sometimes long, periods of PTSD from these encounters.