Thanks for a logical explanation, GGG. I have one PS3 steamer, which I have never opened. Are you saying that there is no internal harness?
MTH did not make the PS-2 3V upgrade kit for steam with production features.
Are you speaking of those features made possible by the addition of a mux board in the engine to control marker, boiler, cab, etc. lights? If so, cannot one be added when upgrading?
[By the way, I'm Bob, not Ralph.]