We saw these train cars in Carlton, MN and were puzzled. It is unlike any train I have seen before. There was a sign on the side of the train that said something about grinding rock. ????? Does it grind up rocks by the ties on the track? We mused it might be a mining train (this is iron mining, taconite country). Obviously, it needs to run on tracks so you couldn't just chew through rocks in the country side. I couldn't find anything online about this. Anyone know?