I did flip the 3r/2r switch back and forth, it worked a little better but only for a very short period. That leads me to believe the switch is the culprit, as you suggested. I did not get conductivity between the drive wheels with the 3r/2r switch in either position. I have not pulled the shell off the tender to look at the switch. I think it may be more like a 5 cent switch. Is the 3r/2r switch a SP/ST or DT or DP/ST or DT? There are some very good relatively inexpensive toggle microswitches out there that could be installed in a different location in the tender. Since I'm only using 3 rail, bypassing the switch altogether may be a best option. If I understand correctly, only the 3r/2r switch needs to be bypassed? That should be a good and reversible fix for 3r operation.
I also placed a temporary jumper between the tenders common copper brush hold down screw and a screw on that truck near the brush. Photos attached.
I then get great conductivity between both sides of the drive wheels on the locomotive with the jumper. There is a spot on the tender common pickup truck to route a short stiff copper wire between the brush mounting screw to a truck mount screw where there will be no wire movement caused by truck rotation. Do you think that should effectively bypass the 3r/2r switch? Do you foresee any problems with making that more permanent, as long as the "jumper" is mechanically protected (i.e. tacked to fixed points on the truck so it won't catch on objects? Very small ring eyelets with the jump wire soldered to the eyelets should make good connections and not protrude beyond the brush mounting screw. My rough mockup shows an 18ga magnet wire in a rough position just to get the idea across. I don't know how reliable the connection between the tender truck and chassis would be. It would take more creativity to do the same for the locomotive common brush, since that truck appears to plastic.
Another possible solution to throw out:
Will a standard 3rail, non isolated MTH freight truck wheel/axle set fit in the tender and locomotive trucks? If so, that should help insure good connectivity as they pass over isolation track sections and switches. If that can also work, I just have to find some sacrificial wheelsets.
