gunrunnerjohn, there are 12 insulated wheels on my steam engine. I didn't pull the wheels and I made some ASSumptions.
The wipers on the rear axle are wired to the common bundle (I assume that is in contact with one rear wheel).
The driver wheels (one has a rubber tire) on one side appear to have wired contacts to wheel hubs that connect to the common bundle. The front 4 wheels have no way to connect to the common wire. I have no idea how the engine picks up positive and negative power when running on 2 rail. One contact is obviously the common wire bundle. There is no electrical contact/ground to the engine housing.
See instruction sheet page 9 for photo of wheels