Toy Train with Independent Wheelbases Fits Brio and Other Tracks

Working on a high quality printable train. Will require some non-3D printed parts. This train is designed to be compatible with Brio and other wooden train tracks.
This is still in beta. I'm hoping to garner interest in case anybody wishes to help.
My end goal is to design a complete replica of the Polar Express.
UPDATE: 10/4/2014
-
I went to a local hobby shop and found 1/8" brass tubing. It's 3.125mm so pretty close to the 3mm I was planning for. I designed the openings to be 3.5mm