....which is shown here on the upside down chassis. The top side of the metal bar (which is the bottom side on the upside down photo) has to be bare metal. The flat part of the chassis base under the bar in the photo should also be bare metal. Many later models had this painted for some unknown reason. Two of the main drive wheel have rubber tyres fitted and do not pick up the negative connection reliably. The loco relies on the two front bogie wheels on the left to provide better continuity. Difficult to clean off, but I found a scalpel is good for scraping to the bare metal, finished off with fine emery to make the surface smooth again.

When putting the bogies in position, offer the forward end so that the strip or the tang fits between the chassis and that bar.