From Downieville, it's a singletrack climb, then a fast and fun descent, followed by a road ride return to Downieville. Or return on the trail.
Watch your speed on the descent as there are some places where you could slide down an edge.
The trail could be an out and back if you don't want to return on the road. If you do, be on the lookout for descending riders.
Parking is about 1/4 mile behind the Two Rivers Cafe. The trail begins with a double to singletrack switchback climb for about 2 miles, narrow in spots. You are nearly immediately in tree cover and remain there for the rest of the ride. There are a few very steep and switchbacked climbs, and there's about another mile to climb after you cross the wooden bridge.
Don't be discouraged, what follows is a fast and flowy singletrack descent following the contours of the mountain back down. It's mostly smooth singletrack with a few small sections requiring technical riding skills, but not many. Watch your speed on the descent as there are some places where you could slide down an edge. Overall, it's a bit more than "Intermediate", but a bit less than "Difficult".
Once you finish the trail, follow the fireroad downhill about a mile to the first bridge. Cross that and follow Route 49 (direction => up) about 4 miles back to Downieville, or turn around and head back up the North Yuba Trail
. It's not a heavily used trail, but be on the lookout for descending riders if you do.