It’s a state park – the Milwaukee Road right-of-way over Snoqualmie Pass. It’s been described as the longest rail-trail in the country. You get to go through tunnels and over tressles. The grade is really gentle. East of Ellensburg it gets a little sketchy. For biking purposes it ends in Vantage at an abandoned bridge over the Columbia…
Here’s the Wikipedia link about the John Wayne trail. Here are interesting articles about Cedar Falls and Snoqualmie Pass.
Here’s the State Parks brochure. Here’s a link to their (rudimentary) website. Here’s a BikingBis article about the trail
Here’s a Seattle Bike Blog announcement from 2013 about the reopening of the tunnels east of the pass.