Carbon River Road
Mt Rainier Gravel and Dirt
The Carbon River Road goes through the only rainforest in the Puget Sound region.
It has been closed to motor vehicles since 2006.
The problem with a rainforest is that it rains--

AND RAINS.
Now the five-mile route is part road, part trail.
It is a bit rocky in places,
and muddy in others.
The changing course of the river made it economically prohibitive to keep open for cars,
but made for a great trail for hikers and bikers.
Like the Westside Road, you can combine your ride with a hike.  There are racks for locking your trike, although there are likely few thieves venturing the five miles to get here.
No cycles allowed on trails in the park, but you would have a hard time riding this one anyway.
A 3.5 mile walk rewards you with an up close view of the Carbon Glacier snout,feet the lowest elevation glacier in the lower 48 at 3500 feet.  On the way, you cross a flexing suspension footbridge.

(See the Westside Road for an even bigger suspension bridge.)