Was there anything the photographer could have done to prevent the accident?
Could he have stopped the train? Could he have run out and tackled the guy to the ground, knocking him out of harm's way? In doing so, would he have put himself in danger of being killed? Would yelling have stopped the guy from running out into the tracks?
Or was it a done deal?
So, then, taking a photograph of an accident would be documentation. Wouldn't it?
What if the victim's body was mangled beyond recognition? A photograph and the photographer's testimony would certainly establish his identity.
Could a photograph help determine whether this was an accident, a suicide or even a murder?
Is there a way this could have been prevented? Photographic evidence could be useful in determining that.
So, if you are at the scene of some tragedy and there is anything you can do to help, do so!
But, if you are witnessing a fait accompli, maybe, just maybe, taking a photograph is the best thing you could do.