It looks like it would be fun. Sucks that they say they aren't supposed to go over 10MPH though. Think of the low friction you'd have on the straights!
On the other hand...this wouldn't work out here in MN very well. There aren't really any abandoned or rarely used tracks. BNSF and Soo Line use up all they can. Plus I find it hard to get permission from BNSF to use their track. My father and mother both work for BNSF and I have talked with many people that work on the rails.... I bet they'd crap their pants if they saw something like that on the rails, lol.
This area of MN has some decent bike trails though, so I guess that would be the MN verision of the rail riders.
I've always dreamed of making one of those. If you built a aerodynamic recumbent, one could go pretty quick!
I've also dreamed of putting hydraulicly lowered train wheels on my travelling vehicles, so I could drive the rails, and be able to drive off of them easily. Imagine going 150mph in a sprortscar on rails, or driving a Winnebago on the rails... I bet you could easily get a small car to get 100mpg that way. You could cut into the wheelwells, so the car is actually lower than the rails.
When I drive downtown, I always drive on the light rail lines when I can. It's soo smoooth! My car takes off jerky, so I have to be careful to not spin the tires.