First, I printed the motor carrier, then gently file the inside channel until it moves freely on the track. 3d printed parts were printed using PLA at .2mm layer height and 30% fill.

Add the wheels using 3mm x 8mm screws. The wheels should turn freely, but not wobble.

The gear should be printed at 10.5% of the original size. After inserting the motor from the top, press fit the gear onto the shaft (note the flat notch to hold the gear in place). 

The gear diameter is 7 mm--making it the smallest functional gear I have printed.

The motor assembly should look like this.

Place wheels on the carriage assembly.

Place the body (large hole end) over the carriage assembly shaft.

Thread the printed nut onto the carriage shaft so that the body can move freely.

Repeat for the other carriage. Attach the two carriages to the motor using the 3mm x 12mm screws. This should allow the cars to move freely. Note that the cars are tilted slightly toward the motor so that the weight of the cars will press down on the motor and hold it in contact with the track.

Electrical connections should be made according to this schematic diagram.

I glued the reindeer atop the cars and attached wires between cars using servo motor cables.

If you don't happen to have old Lego monorail track, you'll need to print your own. I provided the track for people who choose to make everything themselves. The track connectors are somewhat crude--you'll have to use a 3/32" drill bit and make holes to thread the 3mm x 8mm screws through the connector into the track.

I created a very short piece of the geared track (called "track piece") that has a few teeth. In Tinkercad I duplicated this several times and combined the duplicate pieces to make the straight track. For the curved track, I duplicated the original piece (then moved it to the right and aligned the teeth). Next I rotated the duplicate piece three degrees. I did this fifteen times (45 degrees) and that makes a "curved" piece.

The undecorated train looks like this.

Operating on the printed track, Rudolph and Bluedolph look like this.