Going back to the original question posed you don't need to worry about isolation pins or whether the outside rails are tied together. Once the signal gets out of the command base, it pretty much goes everywhere.
SNIP...
OK, take about two to three feet of bell wire and attach that to the antenna terminal on the command base but don't attach it to anything else. Just leave it so it doesn't make contact with anything. You should now be getting TMCC signal. Maybe not great, but it should be there. (This is the flip side of the TPC test. I tried disconnecting the wire on the track side and loco's were still seeing the command signal. The 18" of hookup wire was enough for the signal to break out).
Hijacking but it's pertinent...
I have seen more than one Legacy base with a very much reduced U terminal track signal. About 1/6 th or less than a properly working one. The problem with this is that the Lionel Service department has pronounced this base as working properly. But in the owners layout, it is very iffy...works and doesn't work. As you might guess, they didn't measure the level of the track signal but just that it worked on their test layout. It also worked on the owners test layout, but not well on his big layout. The base was clearly "broken" but there was enough track signal to confuse the issue.
