Is there a consistent trail rating system? I have often seen different guidebooks state certain trails are "easy, moderate or difficult" but I have no idea if that is consistent for trails from different areas. I also do not know what the ratings I see necessarily mean.

When I used to rock climb, it was pretty easy. If I was fairly consistent at flashing 5.8s and could work out the 5.9s I could also usually safely challenge myself on 5.10.

Is there a rating system based on elevation gain over given distance and allowing for some other factors (terrain, weather extremes, trail treadway)? Obviously long trails could be rated for various sections. On the AT, sections in the Whites would likely have very different ratings than sections in MA and sections in PA.