Do we know that his unloading car can operate with "hot" power coming from the 4th and/or 5th rails? These cars pick up the "common" from the outer rail via the wheels.
In your diagram, when the controller right side button (UNLOAD) is pressed "down", "hot" (red) goes to the right control rail and "common" (black) goes to the left control rail. These are for cars that don't require an electrical connection from the wheels and get their "hot" and "ground" from two slider shoes that go to the 4th and 5th rails. Hence the car must be centered over the control rails so that one shoe picks up the left rail to get "common" and the other shoe picks up the right rail to get "hot".