Underside of Lionel No.4 showing one of the two cast iron weights. Its likely that the weights were designed to add traction. A similar weight is affixed to the opposite end of the locomotive. I have also seen these weights mounted over the frame inside the cab.

Part of the Bild-A-Loco collector shoe is also visible. A feature of this design is that the collector shoe can easily be removed. With the collector shoe removed the axles with their captive bushings and pressed on wheels can be lifted out of the motor bearing plates.