Could it be---Alexandria to Roslyn in DC? or The Bike and Hike Trail in Cleveland that opened up in 1972? Or the trail that Follows Chicago's waterfront that was old railroad wharves on Lake Michigan that was done in the 1960s
I guess it depends on whether you mean 'official' rail trail. (meaning state or local gov't funded or approved)
Many abandoned ROW's became 'trails' just by being abandoned, and then used by local residents. For example, many parts of the North Jersey Rapid Transit ROW in NJ has been used as a trail since it's abandonment in 1928. Many parts of its ROW still are unofficially used as 'trails'.