Mike, here is the RS 5.5 board that is in that engine. Maybe the motherboard is new but the boards that plug into are all early legacy. R4LC, Audio power board and this Railsounds board.
Pete
Thanks Pete for the clarification!
Here's what's clearly not obvious in the documentation for this engine: Unlike every other wiring diagram I've ever seen from Lionel, e.g. those in Supplements 1 - 47, this one doesn't show that the motherboard has other boards plugged into it (although the term "motherboard" would lead one to believe that it does so I should have assumed so).
Before I posted I did try to locate the motherboard itself in the Lionel parts system in order to add more light, only to find that it's not actually in the system.
Lastly I checked Rod's summary of motherboards and didn't find it there either:
Updated Revised Version of Lionel Motherboard Connections PDF Available | Rod Stewart
It just goes to show that posts in this forum, and people like you on it, are critical at times for determining the real answer to many questions, especially when the formal documentation occasionally comes up a little short.
Mike