Same from me.

And even 100 km per hour may be too fast. Just consider the dust flying off of an open box car. I think that 60 km may even be sufficient.

Cost of transportation of the ore is paramount.

Next, environment.

Then safety, no derailments. The higher the speed then more likely.

Speed is at least fourth if not further back. Slow enough until the speed is too slow and impacts on costs. So mostly operating costs would determine train speed.