Jon, when you say "dump solenoid" are you referring to the solenoid that operates the two small chutes that dump coal into a waiting hopper ? I noticed, while testing the piece, that these chutes are not opening all the way. I tried more voltage but they don't open enough.
On the issue of the left hand brush holder. Are you speaking of the carbon brush holder on the motor itself ?
I might need a wiring diagram or photo to grasp what you are trying to say.
As for the Williams dump cars, they do operate on track power, but they don't need a UCS activation track to operate. They can be triggered anywhere on the layout.