RAIB don't investigate all accidents, only ones where they feel that there are important lessons to be learned to make the railways safer.
Accidents do happen, not always someones fault and there's not a lot you can do about them.
Two things:
Firstly, my suggestion that it might be reportable to RAIB came from RAIB's own guidelines - they don't investigate
everything (suicides and trespass injuries aren't even reportable - again from their own guidelines), but a derailment blocking an open running line
is reportable to them. I don't suppose that necessarily means it'll be investigated, but that's not what I said.
Secondly, all accidents have a cause - it might not be directly caused by the actions of anyone, but it could be a lack of maintenance or poor design for example. Even where it's down to flooding, a landslip or similar, it may be that the accident could have been avoided if the response had been more efficient - something that will come out in the investigation. So there's almost always something to be learnt, even if that's done via an internal investigation (say for a low-speed derailment within the confines of a yard) rather than the RAIB.
