Too much of a good thing? I'm referring to the TMCC "ground plane" transmission wires next to the track. I would first disconnect that wiring from the AC outlets ground and re-test Legacy with only the track outside rail wire connected to the base. The locomotive gets its radio signal in two parts: (a) the rails via the wheels, and (b) the antenna over your layout - i.e. your house wiring's ground wire. If one part overwhelms the other, such as when the track signal is also radiating above the locomotive while it is under bridges and upper levels, the locomotive can't receive the signal.
With the TMCC "ground plane" disconnected, verify that the wiring is completely isolated from the track. A single connection to the track defeats the entire system and causes problems similar to what you are experiencing. I would not reconnect the "ground plane" wiring until Legacy works without it, and then reconnect only if you have weak signal (the loco's headlight flickers) in parts of the layout.
Verify that the Legacy power brick is connecting to the house wiring (the external antenna) ground. Use an outlet tester to verify the outlet you are using is wired correctly (I'm surprised at the number of times I've found this is the problem).
The SER2 6-81326 is not needed unless you are using Lionel's LCS system.
