At a local park I've seen hiking/mountain biking trails marked with a green circle, blue square, or black diamond representing easy, intermediate, and hard trails. I've only ever seen this while hiking in this location and wonder if it is displayed for the mountain biking rather than hikers, although the system seems to hold true when hiking.
I came across this nationalparks.fi site which has a similar system for hiking trails with a blue circle for easy, pink square for intermediate, and black triangle for demanding with a short explanation of elevation gain and terrain type for each.
Is there a similar system used in the United States for rating the difficulty of hiking trails?