The 403 trail is only a few miles long, not a ton of singletrack for the entire approx 26 mile loop, and the trail is in fair condition is some places. It changes from very technical to fast rolling singletrack and is overall fun. Don't bother with cars and crap like that. Take Washington Gulch road from 135. There's countless great rides here.
Locals keep the secrets to themselves.
Trail 403 itself is short. Though technically challenging for the best, it is eroded and needs some work. It is off-camber and quite steep in spots. It dumps you out at the Gothic campground. Snodgrass seasonlly closes for cattle.