I cant reference what you've listed, and I'm not familiar with your skills, so....
Are the boards for a steamer originally? Is a smoke unit involved somewhere??
Here are diagrams that might help here(page .22-?) http://www.electricrr.com/Manuals/RSCmdr-Inst.pdf
If nobody responds with exact diagrams/etc, then photos or drawings may help deliver answers more quickly.
I'll look at it all from the "basics" till then.
The switch? A slide(direction on/off or program(optional)
The dual black light lead could be a dual color/focus/bright, or a continuation of ground to something else(I.e. extra), or separate leads to the fwd & rev portions of the plug/board.(single light on a directional output) Test this last, and match to direction at that time. Is it a bulb or LED?
Will the tender get a light for reverse? Is the headlight to stay on either direction?
The tether just needs to deliver power to the motor. 3 wires for AC motors. 2 wires for DC motors, [leaving two wires with and/or possibilities. lights? smoke? center rail going back? ground tie) Lighting power can be grabbed off the motor or rail directly. One wire on the tether is likely for front center rail engine rollers to deliver power to the tender.