In the Windows version of NoLimts, there was an option to ride the coaster in the editor. When you go to 3D mode, you can freely move around the enviornment. There was a setting that by hitting "w", you would move forward on the track, and "s" would move you backwards.
I found this screen shot in the OSX help menu, which is exactly what I'm looking for. I just can't seem to get the key combination right to launch it.