These tracks were put online quite a while ago so they're likely not all online anymore. You might still be able to find downloads for some of these by searching online. To play, download it, unzip, and add it to the folder Proun/Tracks.
Modders can create new tracks for Proun using 3D Studio MAX! Check the documentation in the folder Proun/Exporter. Here are some of the coolest tracks that community members have made:
By Chris Farrell
By JohnArr
"Solar Coaster"
By tpapp157
"Smurfland Sky"
By dll32
"Shape Factory"
By Bigguy1175
"Cable Rats"
By Seikusa
By Yooby