Marv,
River Bridge was the first bridge out of town. It is about 7 miles up the ROW, most of which is still there and a lot of Rail and Ties are still in place too. When the "West Side and Cherry Valley Railway" got started they would run trains up the canyon to end of track which is just about at the former site at River Bridge. Well Bell took over they stopped running trains up there and laid track around the Mill Area.
Worse case you can hike the ROW out of town and you will end up at the River Bridge Site. It will be obvious when the ROW drops off. The lumber from River Bridge was used to build a lodge close to town year ago. So I guess you can go visit River Bridge, or at least the lumber that was used.
Kevin Cook