The Illinois Prairie Path is a system of old railroad tracks that have been covered over with crushed limestone. You can read about it here:
Personally, I prefer cycling on asphalt trails. Crushed limestone (or "limestone screenings") misbehaves after it has gotten wet. Parts of the trail get washed out which is a crash hazard. Horse's hooves become imprinted in the trail and this makes the ride bumpy. But it's fun riding with my sister!
Rhyme of the day:
I avoid limestone
When riding alone.