Not that many years ago (post-2000) they still ran tourist trains up that line, starting in Renton and going to Kirkland...
The Kirkland section of the trail actually gets a fair amount of use as a bike and walking route and connects sections of Kirkland that weren't easy to connect before. With the South Kirkland park and ride to at the south border I know a few people who used it to bike to the bus system.
Kemper Freeman I think has a lot to do with why the train line wasn't used for light rail.
That's the problem with turning it into a bike path. Sure, it's nice to have a bike path. But a bike path moves maybe 1% of the commuters that light rail would.
A bike path minimizes utility, rather than maximizes it.