Rails to Trails Conservancy   is the best source of information about rail trails nationwide. 

Trail Link provides current information and basic maps for trails including rail trails. You can see all of the information with a free account. There is a mobile app as well.
If you use the site often, it is worth the annual fee for the upgrade to Unlimited status which has more features.

George S. Mickelson Trail in the Black Hills of South Dakota is a member of the Rail Trail Hall of Fame.  It is a 110-mile long South Dakota State Park. 

Weiser River Trail in southwestern Idaho is owned and managed by the nonprofit Friends of the Weiser River Trail.  It is one a only a few rail trails that are privately owned.  

Trail of the Coeur d’Alenes in northern Idaho is a 75-mile long paved trail suitable for road bikes.  It is owned by the Idaho State Parks.

Route of the Hiawatha in northern Idaho is a 16-mile long trail that has a 1.3 mile long tunnel and great interpretive sites of the railroad and 1910 fires.  It is owned by the Forest Service.