In summary, there are several approaches mentioned above all of which will work:
1) Add a second transformer to provide 14 VAC accessory power. The best and a long term solution. I used a 1033 with my KW.
2) Run the Banjo off track power, the A-U or A-B solution. Lionel says the Banjo works on 12-18 volts. Banjo performance will vary depending on the voltage. I would recommend testing your Banjo first by varying track voltage to determine the range of track voltage that yields acceptable performance.
3) Run the Banjo off the same transformer power as the other 3 wire accessories. Just guessing, but this probably is D-U. Maybe the Banjo operation will be acceptable to Mike23. I would expect the Banjo will run too fast but maybe not ? If to fast, the question is how to simply cut the voltage down. (serial diodes, resistor or lamps)
There are still other more complex solutions which use KW C-D 14 volt power, like a relay or a track side IR sensor.
KW C-D power cannot be used directly with an insulated track trigger for reasons mentioned in this thread.
