I know the feeling
Maybe this will help;
First is a schematic. This is based on hours of tracing out the circuits. Bachmann has a similar one, but it has a couple errors, and this one lays out the circuits in a way that makes them a little easier to understand. I've had a few friends check this one and it seems to be correct.
Click on the drawing to load a bigger one.
Second, here is a corrected interconnection drawing based on the one that should have been included with you Kay. I only included enough parts of the loco to identify the board locations. I corrected some glaring errors in the wiring, and color coded the drawing to match the actual colors of the wires.
You can click on this small drawing to download a larger one.
Dave has covered may of the things you might want to improve on the Kay. However, He runs battery power. If you want to run the Kay on DC, there are some other things to consider;
Bachmann's drawing shows a pair wire connections between the tender front and rear trucks. They don't exist. Look at my drawing and you will see that if the left wheels of the trar truck ore on opposit polarity track from the front track, the there will be a direct short between the front and rear truck. IE. as in when you split a turnout. the entire maximum current of you power supply will then travel from the rear truck to the circuit board, through the lands of the board and out the front of the board to the front truck. If you are using something like a 15 amp pack, expect to buy a replacement board. I recomend a little upgrade, cut the wires from the rear truck right where they enter the rear of the circuit board, extend the wires and splice them into the wires coming from the front truck. While you are at it, examine how the wires going through the floor of the tender to the trucks, have to go through emall holes in a METAL bolster. The holes need to be enlarged, the sharp edges smothed, and some extra heat shrink added so the wires don't chafe the insulation and short out on curves.
Now look at the wiring of the loco.
Here is a real challange. This is for fun, the first correct answer wins.
Trace the path of the electricity entering the loco on the RIGHT front driver. Follow it through the front pickup brush board, second pickup board, etc all the way to the dummy plug, and then back through the loco through the motor, and then all the way back to the LEFT front driver. When doing this, count each time the circuit ENTERS a circuit board.
HOW MANY TIMES DID THE MOTOR CURRENT GO THROUGH A CIRCUIT BOARD?
A [ ] 9 times
B [ ] 13 times
C [ ] 22 times
For extra credit:
How many times did the motor current have to go through a connector pin?
Another fun piece of trivia. The current going throug the motor by entering the front left driver and exiting the front right driver travels through over 6 feet of wire.
Of course, the answers to those questions are things you might want to consider if you expect plan to make any improvements to your Kay.