I bought a train model kit for Esther at Christmas. After we painted and assembled it, the motor ran fine, but the wheels stuck often. The mechanism relies on the left and right sides being a quarter turn out of phase, but the nylon connectors often slip on the metal axles, causing the wheels to lock up.

I found a nice solution to the problem. Anyone who buys the MWK-06 kit and has problems with slippage probably ought to do this. The strategy is to ensure the nylon "L" connectors never slip, and I did that by grinding the ends of the axles.

1. Use a small metal grinder (I used a rotary tool) to grind the ends of every axle into flattened surfaces, with the two ends of each axle oriented exactly out of phase with each other. In other words, grind one end in a horizontal orientation and the other end in a vertical orientation.
2. Squeeze the axle shafts on the nylon L connectors using pliers, flattening the round holes and turning them into ellipses. Flatten all of them the same way.
3. Press the flattened L connectors onto the axles. The connectors should fit easily, but snugly; if they are loose, squeeze the L connectors some more.
4. To allow free movement, tighten the screws connected to the pulleys and the second pair of wheels from the back as much as possible without causing friction. Loosen the other 4 screws as much as possible without letting the nut fall off.

I can supply pictures if these directions don't suffice. The train moves pretty fast once it's running properly.