The rest of the story two months later, for the record, in the event this might serve as a resource for someone:
@gunrunnerjohn posted:Well, I'd have to see the "cooked" parts up close to determine if this is really the issue. The board you are indicating is indeed the $120 RCMC, that little top board is part of the assembly and is not available as a separate part.
I'm surprised it runs in conventional if that board smoked, but again this is likely one I'd have to see.
I should have taken the GRJ hint that it should not be running in conventional if the RCMC board was cooked. $120 later, it turned out to be the bluetooth board. I was convinced that as bad as the smell was when it died, I would see external signs of failure, but no joy and it is what it is. What I thought I saw on the RCMC must have been some flux splatter.
The new BT board is on the right, and you can see the wires are not long enough, plus one of the wires should go to the BT on / off switch, a bit of soldering and all is good. This is what Lionel sent as the official replacement part for my loco. Must fit a lot of locos in this configuration and not worth carrying wire harness changes as different stock units. BTW, Lionel's parts shipment was not bad, from order to arrival for each board was about 2 weeks. I guess my parts inventory now has a RCMC board that appears to be good. 
