This abandoned railroad car was probably converted to an office or dormitory for track workers a really LONG time ago (it's on a concrete foundation). And then it was eventually abandoned. It's now appears to be a squat for...urban campers. Alas, they are not particularly creative when it comes to graffiti.
I looked it up on the web and there are a couple of videos and pictures. The car has been really overgrown even in the last few years. Life does indeed find a way.