My interpretatiion of what @Scott Kay was trying to relate to you is that in an individual wheel-axle-wheel, one wheel is isolated from the axle, but the opposite wheel is not isolated from the axle. Thus if in one truck, one wheel-axle-wheel is in stalled with the isolated wheel to the right side of the truck, and another wheel-axle-wheel in the same truck is installed with the isolated wheel to the left side of the truck, a short is created between the two rails by the connection of non-isolated wheel to its axle - to metal truck - to non-isolated wheel of other wheel set.