I recently purchased a USRA Light Southern 4501 DCC ready loco (54403) and will be installing a Soundtraxx TSU2 decoder.  I wanted to make sure I understand the wiring at the existing circuit board.  Especially the B+ and B- connections which I can guess what it might be, and then be wrong.  So here is what I assume about the wiring from the Engine to the Tender.

R = Right Rail
M+ = Motor
L- = Headlight Common
B+ = ??
L+ = Headlight Positive
B- = ??
M- = Motor
L = Left Rail

I would appreciate any corrections and filling in the blanks nad the B+/B-.  Thanks, Mike