There seems to be some different definitions floating around. I consider a module "railroading" (to some extent) when the module writer includes an involved story or plot that moves the characters from point a to point b. This can be forced or strongly suggested by the DM (as it has the same effect). I don't consider a shipwreck module a "railroad" just because they are shipwrecked (without the players getting themselves into the situation.) I consider railroading to be when there is a correct path written in detail to get out with text description of the along the way (rather then a big map of the island and numbers identifying places that can investigate in any order you like or can be skipped all together. In this design, the players make up the "detailed" story as they go.