It's a problem that many former industrial areas have to deal with. It's not just that the former factories have to be brought to a different use, but also the infrastructure that was used has become abandoned. What should be done with all that?
For instance in the Ruhrgebiet in Germany. The area was heavily dependant on coal and steel industry for decades. Most of the different factories and mining plants were shut down because production was simply not economicly sound anymore. Of course, this left an economic crisis to the area, that suffers from high unemployment. It also leads to urbanistic problems: massive sites need to be redeveloped or reprogrammed. But it's also infrastructural: coal, cokes, iron, steel and everything was transported by rail through the area and away from it. The tracks formed a web of dissections in the urban tissue of the densely populated area of the Ruhrgebiet.
Then, at a certain point in time, somebody had a smart idea: let's re-use those railtracks! Former industrial infrastructure graduately became (and becomes) recreational infrastructure. In place of the former railways, bicycle and pedestrian routes have been established, making place for recreational routes in an area that has otherwise trouble creating just those routes. Two problems - one solution for the both.
So right now, there are several routes crossing the Ruhr area, connecting the different sites of industrial heritage. For a map of the possible routes you can check the site of the route industriekultur. Personally, I'm pretty impressed with the structural solutions this brings. And, more importantly, it's great to ride these routes and see the different sites. For instance: the Erzbahn. It starts in Bochum, at the Jahrhunderthalle (the industrial steel-manufacturing hall that's transformed into an event space. And it runs straight through Bochum, to Herne and Gelsenkirchen, to end in Herten. Along the route there are several industrial monuments, such as Zeche Pluto, the Glückauf-Siedlung or the Malakow-tower, for instance. Other than that, the Erzbahn, which is several metres lifted from the ground, offers a great view of the cities and the landscape. Also, the former railway-bridges offer a special thrill, like the Pfeilerbrücke: a 45 metres high, 340 metres spanning bridge. In other places, new bridges have been constructed to connect "missing links" in the system. The entire trajectory gives the pedestrian or cyclist a whole new perspective on the area.