Since it's close to me physically I got a little interested and did some digging around with people my boss (donor) knows...
Two of the cars are sitting in Hawarden, IA. They got repainted and were used for a "Polar Express" type of holiday ride. From what I'm told they only got used for that once because they didn't make any money from it and they're just sitting in a rail storage yard over there. For context, Hawarden is a less than one horse town about 30 mins from me right on the Missouri river.
Sounds like the Hawkeye Express doesn't exist anymore outside of that one lonely car sitting in Dell Rapids rotting away. There were 6 cars total, so even if they wanted to use it again someone would have to buy back the remaining 4 cars, (if the 3 unaccounted for even still exist), repaint, and get them going again. You couldn't just have one or two cars running because there's no way that math works out.
I'm guessing with the kinds of winters we get 'round these parts that none of them are in good shape or being maintained. I'd hardly think whoever owns them is doing anything to winterize/maintain them.