I think I should call Mulder & Scully in on this little entry to the X-Files: invisible motorbikes on the Railway Path. As you can see from the picture, these 4-inch wide tracks appeared from nowhere, their makers invisible to the eye, like that creature from the Id scene in Forbidden Planet (fab movie, get the DVD). My own theory is that it couldn't possibly be numpties racing mini-motorbikes along the path, but actually a 15 foot tall mountain biker with stupidly wide tires.