I had the same query when I first found Simtropolis. After quite a bit of digging in the forums, I found these two threads. Could be useful.
1. Without curves
https://www.simtropolis.com/forum/messageview.cfm?catid=23&threadid=75821&STARTPAGE=3&FTVAR_FORUMVIEWTMP=Linear#1340077
2. With curves
https://www.simtropolis.com/forum/messageview.cfm?catid=23&threadid=75821&STARTPAGE=3&FTVAR_FORUMVIEWTMP=Linear#1340078