Raging River is a baller trail system on the north side of Rattlesnake Ridge just east of Seattle. Obviously most people drive there but I'm not really into that. You can access the north side from at least two places. The main lot or Rattlesnake Road. In either case, from Seattle, you'd need to ride to the area south of Snoqualmie and then cross I-90 one way or another. It looked on the map like the logging roads that are all over Taylor Mountain, just southwest of Rattlesnake might allow you to enter Raging River from the south, either by the fire road to the top or via the climbing trail. Unrelated but I was super creeped out when I read in the Stranger that Ted Bundy used to dump people out there. Riding it at night on a weekday was a cool experience. Completely empty.
It seems like there are multiple legal and many more illegal options to make it over Taylor Mountain. The one I took here is to ride east on SE 208th (in Hobart). It is a series of rollers which starts paved and then to gravel (but well behaved). Eventually this dead ends into private property, and on your right is the Cedar River Watershed, which is a no-go area. Right before the dead-end there is an unmarked gate to your left. Another option is to start from the Taylor Mountain lot, climb the Holder Ridge Trail and then descend Road G until it intersects SE 208th. To get to Taylor Mountain I'd recommend taking SE May Valley, Cedar Grove, Lake Francis, and then 200th. Another good option is the Cedar River Trail, SE Jones Rd, climb 196th, and then the very nice Sweeny Rd. Anyway, this is what the connector looks like.
The road is much rougher pretty much all the way until you connect with the aforementioned Rattlesnake Rd.
Turning right on Rattlesnake Road takes you all the way to the top of Rattlesnake Ridge. You can also turn off and take Return Policy to the top.
Once at the top you can take the main trail which is the buffest singletrack I've ever ridden. Eventually you'll either have to turn onto the rough fire road to get to the bottom or take Flow State.
From the parking lot, to get back to Seattle I rode to Snoqualmie, then up the Snoqualmie Parkway on the bike path, and then from Snoqualmie Ridge took singletrack to the Issaquah-Preston Trail, which takes you bake to Issaquah.
I think 38mm tires are probably a good minimum. That is what I first rode it with and in good conditions it was alright. I'd have been a lot faster and more comfortable with some fatter rubber though.