Nothing to ponder. Each transformer you listed creates the DC offset differently using different circuits and methods which can result in differences in DC offset.
The difference between loaded- moving and not loaded- neutral was right on the threshold of detecting the DC offset voltage or difference.
According to a search at Lionel Support, and Lionel, I'm not 100% sure what exact engine you have. One search says you might have a Conventional classic that only has horn and bell http://www.lionel.com/products...v-loco-2328-6-31775/
Another search came up and showed a Railsounds 3 with that road number and type as a PWC series.
https://www.lionelsupport.com/...MCC-GP7-2328-6-18892
If your loco is the PWC version and has Railsounds 3.0, which IMO is one of the worst series they ever made. The power board for that 3.0 set has some stricter input requirements and the amplifiers on the Railsounds 3 sound board are also not my favorite. I often transplant the sound chips to Railsounds 4.0 series boards and use a Railsounds 4 universal power board if possible when the motherboard supports that change from Railsounds 3 to Railsounds 4.0.
