For the above to happen you would need a series of events that is hard to imagine. First the trim motor would need to begin a runaway. As soon as that happened the autopilot or pilot flying would counter that with elevator movement activating the trim brake. If the trim brake failed which would be a completely different failure from the initial runaway the pilots would need to not notice the autopilot disconnect warning, nose pitching down, change in G forces, spinning trim wheels. If any off the above was noticed the pilots would simply hit the trim disconnect switches. They would also intuitively trim counter to the runaway. Either action should stop the runaway. They could then continue the flight using manual trim. The trim brake is a robust system we checked on the first flight of the day so it’s tested often.