Also, the longer the rails have existed, the more likely that many other smart people have tried and failed. It doesn't mean it can't be done, just that it is

Also, the longer the rails have existed, the more likely that many other smart people have tried and failed. It doesn't mean it can't be done, just that it is likely very difficult, or some other tangential breakthrough must happen first. Fortunately, some rails are far less well-defined than others, and there are often nuggets of gold found in the foggy ambiguity surrounding such rails.