What is a defect that requires a train to be notified and stopped?

A defect that requires a train to be notified and stopped includes all the scenarios presented. Each of these defects has the potential to create unsafe conditions for both the train and its passengers, necessitating immediate action.

A hot journal indicates overheating in the wheel assembly, which can lead to wheel or axle failure if not addressed, posing a severe risk during operation. An open plug door can compromise passenger safety by potentially allowing unauthorized access or endangering individuals who may inadvertently fall out while the train is in motion. Lastly, a broken wheel can lead to derailment or catastrophic failure during transit, making it essential for the train to stop immediately to prevent accidents.

Therefore, the most comprehensive answer encompasses all these various defects, emphasizing the importance of stopping and notifying the train to ensure safety is prioritized in all circumstances.