Complex sentence: The scene in which a train derailment is observed was not part of the original shooting script, but it is in fact a real train that derailed and when the director heard about this he wanted to incorporate it into the film.
Split into simpler sentences: The scene in which a train derailment is observed was not part of the original shooting script, but it is in fact a real train that derailed. When the director heard about the wreck, he wanted to incorporate it into the film.