Just got back from riding the rail trail from Slocan to South Slocan on our bicycles.  It's a nice flat 50k ride down at maybe 1% grade in most places.  Some nice views of the river.  The trailhead is at Slocan, an old mining community.  Not much there anymore, but the RV park is very nice!  We stayed there two nights at $35 per night with power and water.

The trail is in various states of repair.  Some sections are fully covered with pea gravel.  Others are two hardpack trails with grass in the middle and on both sides.  But everything is quite rideable, and the last 4k into S Slocan are paved.  

Edited to add that there is cell service in Slocan and South Slocan but nothing in between.