The KW has 6V,14Vand a 20V constant. 6V hook up is U post plus 1 D post. To get 20V you use the other U post and the other D post. To get 14V you use both D posts. Unless someone tells me different.
So, are you saying it CAN be done? Somehow separating the 14V constant to the accessory AND allowing the train to cross the track at any/variable speed?