Not sure how common this is in other places but in Massachusetts a fair number of rail trails have sprung up over the past few decades--often over considerable opposition in some town or another. Some, like the Minuteman, were explicitly "bikeways" (although pedestrians and, for a time at least, rollerblades are also common). Others have always been pitched as mixed use.

In any case, it's pretty much a non-starter to prohibit at least conventional bikes or pedestrians even though, on the more crowded and narrower paths, I'll avoid them during busy times as a walker.