In Australia, as in the UK, they hold what is called a Coroner's inquest that when deaths are involved they call witnesses with the intent of figuring out the cause. They use it to determine something like if a death was an accident, suicide or murder, and in the case of an accident like this they will call experts in and the Coroner will determine from that testimony what the root cause was. Ultimately the call is with the experts, the coroner's judgement in most cases is pretty much a rubber stamp from what I know of it.
As far a sand causing trouble with the signals, only thing I can think of is it somehow insulated the wheels enough from the rail that it didn't complete the circuit that would indicate the block was occupied,from what I recall of train signals it works exactly like a detector track on 3 rail, that if you get continuity between the 2 outer rails it causes the signal to indicate red. I don't think sand could interfere with more modern signal detection, but maybe one of the experts on here knows more, I am just guessing.