Old logging road that branches to narrower paths. Much of it would go further if you're willing to cross small streams. Mostly easy trails that can be tackled with even a gravel bike but some Rocky sections need a true mtb to navigate or just be walked. The trails aren't maintained at all so on your first run down a trail keep an eye out for branches.
Trail is open to snow mobiles in the winter, so while a ton more trail opens up when the steams ice over (including all the way to the power lines between 125 and 4), fat bike riding could be dangerous.