As an experiment, Phil disconnected all the "third prongs" from his power source to the TMCC wall wart, making it 'ungrounded", just the current and common wires connected, and it all ran fine for his 1500+ foot railroad. TMCC does not need to be connected to a ground to function correctly, at least on both our layouts.
Additionally, our TMCC "brick" and DCS TIUs are all buried under the freight yard, which is basically covering it with a screen of eight tracks, and we don't have any reception problems. Might move them up into the mountains when that scenery is put in place, but don't see any real need to as long as we don't have signal reception problems.
Going to string one wire under the roadbed along the entire upper loop, although it probably only needs a "ground plane" where one track is above another close by, or passing over a lower track, and connect it to a separate ground rod outside the house, to suck off that excess signal.
We've been hammed into the corner that all seems to work well as long as we keep it simple and close to basics. Going for the enhancements seems to make it work "less than mo betta" ...
Also regularly operate DCS and TMCC trains together under the seperate systems, sometimes on the same track, with no problems. The TMCC engine responds to the CAB1, and the DCS with the handheld. Don't use the "TMCC under DCS" feature. One does not confuse the other.
Every layout seems to be different in that somethings which work fine on one don't do so well on another.
Everyone's mileage will vary of course!