Thank you, Marc
I may end up hard wiring, and not use the plug-in.
the motor wires are clearly defined, I will report how it turns out, probably in several days

Does the circuit board wiring need to be cut anywhere when plugging in a Digitrax decoder SDH164D?
I suspect so, but the instructions from the box (neither loco nor decoder) are not clear ro me.
Also, where can I get a schematic drawing of the circuit board involved  (HB823X PC803) ?