Sorry Dave, no DCS. 100% TMCC.
OK guys, I've done a few of the tests you suggested.
The Rick test: power base, then wait to power track fail
The Brad test: straight Legacy, no "Y" cable fail
The Carl test: straight Legacy, "U" post direct to rail fail
The Carl test, Matt variation: "U" post direct to rail with both outside rails tied together fail
The more I try things, the more I'm leaning toward Dale's diagnosis of a weak output on the base. Although, some of these tests were done with the engine in close proximity to the Legacy signal source. Is it possible that I have something near the base interfering with the signal?