Just found this video on Youtube. I have no idea about the legality of flying 10 feet from the tracks, but this guy was clearly in class G (unregulated) airspace.
However, assuming you could do this legally out in the country (can't do this in cities or near any regulated airspace except with the proper licensing) this might be a good way to follow our favorite railroads, trace old roadbeds, or even to take some awesome pictures.
Apparently, if you're flying a single seat version of this contraption, it is classified as an Ultralight aircraft and only requires a driver's license and a few hours of training. These guys are nuts though as I don't think I'll strap myself to a giant fan and a parachute