You can set up most of the control tracks to work off fixed voltage, not center rail voltage. One of the outside wires going to the controller is coming from the center rail. Just connect that wire to a variable power source. Then set the voltage on that wire to something that gets the dump car to dump properly.
Another possibility is that the insulation on the wire going to the accessory shoe on the cars trucks has hardened. When this happens sometimes the assessors shoe may not sit flat on the control rail. This can cause problems getting the car to dump. These wires can be changed out to super flex wire. This will give the accessory shoe freedom to adjust to the control rail.
