Okay, I got it now, thank you all for the explanations.
Seems a bit odd, mixing both AC and DC in one unit, but I suppose Lionel did this because of their long history of AC-powered stuff...
So the next question is: will I be able to run this engine just using a standard AC transformer? Or will it only operate with that TMCC control system? Judging by Mark's post, it looks like I may be able to just run this, without all those "bells and whistles"...(?)
Mark (this one) in Oregon