Came across this on twistedsifter.com, and then found some more links. Amazing that so much is still extant in a large modern capital city!
There are some lovely brick lined cuttings in places
http://en.wikipedia.org/wiki/Chemin_de_fer_de_Petite_Ceinture
Video of the route made up of stills pieced together (those arty French!):
Not modelling I know, but thought it may interest some