The most fun part about 3D printers is that you can design non existed things. For me, Duplo is a great way to experiment with (And have a happy kid in the process). After the straight rails to test the compatibility and interchangeable with the original Duplo (see: http://www.thingiverse.com/thing:36017 ) I've taken it a step further. And here I present the hexagon railroad crossing. Fully compatible with the existed Duplo rails. The bottom is designed that it fits Duplo in every direction. If you like it and print one yourself, please share a picture. I always find it fun to see others using my designs. The hexagon railroad crossing in the picture's is printed on a Felix in PLA with a 0.25 layer height. But if you want the ribs on the track with more detail a lower layer height is advisable. The Slicer may complain about some hole's or intersecting facets, but I've tested with Slic3r and Skeinforge/Sfact and they both create good G-code. No support is necessary.