The Eastside Rail Corridor (ERC) is an old railway now owned by a partnership of cities, companies, and King County. This multiple use trail provides a nice pathway for hiking and, in some areas, trail running and biking.
More is planned for this corridor including the potential for connecting areas via train. This can provide more transportation options for people to live and work across the Eastside.
While much of this trail has yet to be developed, you'll find a nice open trail with relatively few road crossing and a number of caches along the way.