Regarding the line & its' history: Currently called the Woodinville Sub by BNSF, the line runs from the former NP Seattle - Tacoma mainline @ Black River Jct. ( North of Grady Way, behind Ft. Dent Park ), thru Renton, Bellevue, Woodinville & Maltby to Snohomish, where a connection is made with the ex - GN mainline over Stevens Pass. From Black River to the Boeing spur in North Renton the line is ex - Milwaukee/Pacific Coast RR. From Boeing North to Snohomish the line is ex - NP with various predecessors. The line once continued North of Snohomish to Sedro Woolley, and served as NP's route to the Canadian border @ Sumas. Also, NP once had their own line from Black River to North Renton that crossed the current line in the middle of Burnett street @ the Renton Dinner Train depot.
Current customers on the line include Boeing, Ryerson Steel, & sometimes Dunn Lumber in Renton, Safeway Bakery in Bellevue, & several lumber yards in Woodinville & Maltby.( Don't know about Graham Steel in Kirkland.) The Woodinville Sub has also served as a bypass around the tunnel under downtown Seattle for high/wide loads, most notably the ALCO PA carcasses enroute to Doyleland in Portland, OR.
The Woodinville Sub. will be severed in Bellevue in a few months, when the state starts widening Interstate - 405. The state did not want to spend millions extra to build a shoo - fly around the Wilburton tunnel/overpass section. BNSF agreed, since the line was for sale anyways. (The state says the line will be reopened once the widening is complete; I have my doubts.)
The bike trail suggestion is the idea of King Co. Executive Ron Sims. I guess he never drives I - 405 thru the Eastside during commute times. ( Currently about 18 hours a day. )Perhaps no one on his staff has thought of using the line for Light Rail / Heavy Rail / Busway. After all, the line roughly parallels the busiest highway in the state, per state DOT surveys.