When I built number 10, I included a detailed underframe, complete with boards and the stringers. You can’t see it, and frankly, the tender frame is a bit of a weak spot. Not only that, but I didn’t think about how to get current from the trucks to the decoder until very late in the process, and consequently, there is none.
For 622, I’ve designed a tender frame that is made of four layers of etched metal. The layers of metal give the tender an ounce of weight, and also make a very rigid platform for the rest of the model.
To gather current from the trucks, the two sides of the frame are separated, and each truck is screwed (1-72) into an opposing half. I’ve also provided for two 1-72 screws to terminate decoder power feed wires to the tender floor, thereby making it easier to remove the decoder (take note future self: you’ve already forgotten what those holes are for once!
The tender will be electrically connected to the locomotive through two means. As with #10, the feedwater tubes will actually be fine wire, and will connect the two halves of the frame and the tender. In addition to this wire, the coupling will be a sandwich of metal and plastic. The tender halves will rub against the appropriate face of the coupling to provide the redundant power feed.
Pembroke:87 