So introducing the DPDT switch causes both conventional mode (using Z1000 controller) AND command-mode (using DCS-RC) to stop working.
I'd disconnect the DPDT switch and confirm the two modes still work independently. In other words, the brick barrel plug directly into the Z1000 for conventional...and the brick barrel plug directly into the DCS-RC for command.
If the engine takes off at full-speed in conventional with the Z1000 knob turned down, then the Z1000 may have "shorted" thereby applying full brick voltage all the time.
As I see it, the "no sound" problem should be addressed later. You might have stated in above but also remind us exactly what DCS engine you have (PS2 or PS3, item number).
OK, I have done a fare amount of experimenting.
I also went thru all of my wires and pigtales, and verified that all have good continuity.
1. I went back and tried both controllers on their own, and they work flawlessly, and with all Locos.
2. I hooked everything up without the switch.
Z-1000 Hots connected, with RC Hots not connected
All grounds connected together as before.
RESULTS: Conventional Loco took off as soon as power was turned on, with the Z-1000 knob at "0".
Same as with switch in place.
Based on this result, I didn't go to the trouble to try the RC by itself.
3. I tried again, and this time I disconnected the grounds from either the Z-1000 and/or the RC box.
RESULTS: Definitely better results, but non that made seance, as all seemed backwards.
I can't remember the exact details, so I will try it again tonight and track the changes.
Bottom line is this: I believe that it is the grounds being tied together that is causing the confused behavior when all is connected with the switch, especially when I get the same results with the switch removed, and with the unused Hots simply not connected.
I feel that somehow there is some back feed, via the grounds, so I have a couple of alternatives to consider.
1. Alternate Switch wiring, using same DPDT:
a) Run Power and Ground into both Controllers.
b) Run Power out from both Controllers to one side of the Switch.
c) Run Power out from center of that side of Switch out to Track.
d) Run Ground out from both controllers to opposite side of Switch.
e) Run Ground out from center of that side of Switch to track.
Pro: Hot and Ground completely isolated between controllers.
Con: Power to all controllers at all times, even when not used.
2. Replace 2D2T switch with 3P2T switch:
a) Connect all Power leads, in and out, to Poles 1 & 2, as in original diagram.
b) Connect Grounds, separately, from Brick to each Controller.
c) Connect Grounds out from both controllers to outer terminals of Switch - Pole 3.
d) Connect Ground from center pin - Pole 3 out to track.
1) Complete isolation between controllers and their signals
2) No power to Controller when not in use.
1) More expensive switch
2) More complex wiring.
Again, I will report back on the wired results, when using the original DPDT configuration, and disconnecting grounds from one or the other controller.