a derivative of http://www.thingiverse.com/thing:37104 with a more understandable scad code and more possibility

• get a bigger tooth each X teeth (for bettre grip on snow or mud)
  -possibility of multiple wheels print for more stable large track (as for skidoo)

print the track as 0.5mm wide no infill no top or bottom layer

print the wheel whith 3 perimeters 3 layers top and bottom, 0.2 infill
the same whith the frame

the frame is ready to use these servo: http://www.pololu.com/catalog/product/1248 and is ajustable to get the track with the correct tension

you may need to add an external idler if you use another servo with no ball bearing; i will work on it anyway

work well with slic3r