A simple and probably obvious situation except to me, but why do both wires go back to the circuit board rather than one directly to ground?
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For what engine? For Non R4LC Legacy AC Ground is no longer reference. Like MTH PS-2 Legacy is DC based control. So it needs a DC ground.
TMCC, PS-1 used AC ground as a reference. G
FWIW, even the ground based couplers typically have two wires to the motherboard.
I was converting a Williams PA 1 A-A set to R4LC command with an ERR Cruise Commander, and when putting in a couple Lionel electro-couplers the question came to mind while creating the link to the coupler added to the dummy unit. "Why not just run a one wire tether," I thought? (I ran both.) The answers seem to indicate that depending on the circumstances a board may use AC or DC to operate a coupler. If DC, then a DC ground is needed which is found in the board. So, if correct, why are there two wires to the motherboard with AC, which the R4LC situation apparently is? Aw heck, I don't even know enough to know what the reference to "reference" means? Good grief.
TMCC and early Legacy based on the R2LC/R4LC board couplers have one side grounded (wheels), and the other side powered by the triac on the electronics. Newer Legacy and any MTH DCS locomotive are DC based and MUST have both wires returned to the main electronics board(s). You can simple ground one of the coupler wires to the frame of the trailing A.
What did you do about the lights if you only ran the coupler wires?
Thank you sir!! I didn't put a headlight in the dummy unit. I guess I could add it and ground both it and the coupler in the dummy and use the two wire tether as the connection from the board for each one. That is if stranded 24 AWG is stout enough for the light.
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Speaking of the two wires. That pesky electro-coupler that opens on it's own. I reversed the wires, small modular plug, coupler worked much better. Adding a 50 microfarad capacitor across the coupler leads also helped. Shortest coupler, maybe 1/2 of an Atlas articulated coupler.
50uf is overkill, if you have electrical noise triggering the triac, 1uf is more than sufficient.
@Bucky posted:Thank you sir!! I didn't put a headlight in the dummy unit. I guess I could add it and ground both it and the coupler in the dummy and use the two wire tether as the connection from the board for each one. That is if stranded 24 AWG is stout enough for the light.
I use #30 wire for many lighting chores, #24 is overkill for any locomotive lights that I've ever used. 
Thank you once again.
