Bob, I could not post to your topic for some reason. The 2 black and the 2 red wires on the 7 pin are connected. As John mentioned one is for the chassis ground in the tender.
In installations where the tender has a pickup along with the engine, one red can be attached to the tender pickup while the other goes thru the harness to the engine. Same for the black with chassis ground.
I have seen folks drill a small hole to allow for the marker lights and then use hot glue. Even some Diesels where the shell has a dimple but no markers, you can drill a hole for the marker.
The lights Rod listed are just bulbs and leds with harness. The Steam kit comes with them. The diesel kit has both sets of markers plus the MARS lights available. G