Send the stuff USPS and put a piece of 1/4" plywood on the sides. I think that will be my new shipping standard for anything that is fragile like brass locomotive.
One issue you'll run into with the Williams K4 is the limited space for the tether connector in the locomotive. I have to ditch the standard connector board and use an in-line connector secured in place with JB-Weld and move all the wires to it. Fortunately, there is a matching 10-pin connector available that will fit, it's just a wiring job to move all the wires from the old connector to this one. Obviously, these guys hadn't seen DCS upgrades when they build these. 
