Sounds like the estate engines aren't 100%. First step is to see if there's a continuity issue that when you start to draw power you lose the track connection. Cleaning the wheels and any wheel wipers would be first. Does the smoke work? That would draw enough current to verify if it's the added motor current that is the issue.
Other than that, the fact that they completely cut out as soon as the throttle is cracked tells me either the motor is stalling or the boards may be toasted.
