What must not happen when a 3rd UTO appears on the control board after a train has passed?

When a third unauthorized train operation (UTO) appears on the control board after a train has passed, it is essential for safety reasons to cease all operations in the affected area. The appearance of a third UTO indicates that there is a significant issue that could jeopardize the safety of train operations, such as potential collisions or the presence of unauthorized personnel on the tracks.

Stopping all operations ensures that no additional trains enter the zone where the UTO is detected, allowing crews to investigate and resolve the situation without the risk of further complicating the issue or creating hazardous conditions. This protocol is crucial for maintaining the safety and integrity of rail operations, as it helps to mitigate risks that could arise from a potentially uncontrolled situation.

In contrast, other options such as allowing the dispatcher to start authorizing movements or continuing normal operations could lead to unsafe conditions. These actions would not only disregard the immediate dangers presented by the UTO but would also violate safety protocols designed to protect both train crews and the public. Hence, the decision to halt all operations is a necessary and responsible action in response to the third UTO indication.