We bought some second hand Magic Tracks (aka Flexi Trax aka Clixtracks) kits. It was kind of great, except for the fact that four out of the five cars got stuck all of the time 😱 and they would occasionally run over the side and off the tracks.
Looking a bit closer, the problem was apparent. The cars are designed to steer by rolling the wheels against the track walls, and these original wheels had quite sharp angles. There are plenty of gaps in the track walls for the wheel corners to get stuck in, especially in sharp curves.
This model solves this problem by having rounded outsides. It's a night and day improvement!
I used PLA, 0.4 mm nozzle, 15 % infill and variable layer height. Pretty much anything should work though, except TPU of course because the front wheels require low friction.
My cars have 2 mm diameter axes with some grooves at the ends. 2.22 mm made for a very firm fit: I could push them on by hand, but taking them off requires some decent jiggling or tools. You can tweak this by scaling x & y in the slicer, or in the OpenSCAD source.
The author marked this model as their own original creation.