I've followed it to the end and beyond before. The old railbed goes all the way to Deep Creek where it abruptly ends where a trestle used to be. I hiked my bike across the creek and tried to keep following the railbed and there were a couple of cabooses out in the middle of a field (this was back in the 90s) then I ended up on Clackamas Highway. I used to ride the Springwater on MTB a lot back in the day when very little was paved. I hoped I'd run into a singletrack or two but not much out there, too much private property.
Looks like they're going to make that part of the trail official and call it the Cazadero Trail.
Google Maps link:
You have to drag the map to follow it back up to Boring. For some reason it disappears when I zoom.
(Oops, I see someone else already mentioned the Cazadero.) I think they wrecked the trail by paving it personally. It used to be all gravel save for a little paved bit along Johnson Creek somewhere and a longer paved bit in Gresham (rollerblader/dog walker/baby stroller nightmare!)