This trail runs from Duvall to Snoqualmie Falls and then from North Bend to Rattlesnake Lake. It is on the road bed of the Milwaukee Line and at Rattlesnake Lake it hooks up with the John Wayne trail which follows that same road bed over the mountains (and all the way to the Idaho border.)
North from Duvall the trail continues to the King County line and the road bet continues at least on to Monroe. The stretch between Snoqualmie Falls and North Bend is negotiable, but the roads have a fair amount of traffic. I believe that King County plans to link the two ends together and to link them to the Preston-Snoqualmie trail. Hopefully in my lifetime.
Most of the way between Duval and Snoqualmie Falls is in trees which makes for a cool ride on a hot day. The grade is gentle enough that if you ride from north to south you don’t realize you’re climbing. When you return the first five miles or so go quickly and then it flattens out. The last time I rode it the South end dead-ended at the culvert under Tokul Rd. (There was a path out to the road but it was not easy to ride.)
The parking area at the Duvall end of the trail is cunningly hidden under the bridge over the Snoqualmie River.
Here’s a Seattle Bike Blog write-up from the fall of 2013
here’s a map
here’s an article about a fire in 2014 that closed the bridge at Reining Rd.