One thing to consider is that in any accident the usual cause is almost always multiple things that coincide to create the conditions for the accident to occur.
Human error, loose bolts, one or two mph too fast. rail spread due to heat, sleepers loose, soggy rail bed, uneven loading, one bad bearing, the list can be virtually endless. All the same things may have happened countless times before, one at a time or in a different order and nothing happens, then they align and poof accident.
I think that is why the longer list of things to check. They probably cannot pinpoint one thing as the cause. and those corrections may help to reduce the recurrence.
While the park is responsible, I do not think it was gross negligence per se, just an unfortunate series of events.
my two cents.
Kevin