Update: Finally traced the one white + two red wires that were the output of the obsolete “Normal / Children’s” plug for the DC Junior Engineering Day, and which I removed. I found this:

This exposed crappy-looking connection is key to all DCC power for the Mountain 1 panel. Seriously.

The white wire converts to a black wire that then connects to the main terminal under the Mountain panel:

Once again here we can see all “common grounds” are connected together at the terminal. This also bridges the “common” from both boosters used for Mountain 1 vs 2 directly.

The output terminal had these two red wires. In the picture above, one red wire does indeed go to the Mountain 1 panel and is the source for all the block toggles:

The other red wire goes to the turnout panel (!) to an unrelated toggle named “Front Yard”. That’s interesting as I had tried to understand what this toggle was for, and I think I finally got it: it’s a remnant from the DC era allowing to feed the Stockton Yard panel from one of the Mountain panels:

I added the M1 / M2 markings to make it explicit that it takes power from either the Mountain 1 or Mountain 2 panels and outputs it to the “Front Yard” wire. That wire in turn connects to a power selector on the Stockton Yard panel.

In the end, I rectified this by directly powering the Mountain 1 panel toggles from the DCC input relay, by adding a new wire from the relay soldered to the positive bus bar. This bypasses the “Children’s” terminal and the Front Yard one. It does not invalidate them as they are still connected yet at least they are no longer the required electrical path to power the Mountain 1 panel.

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