What must a train crew do after encountering a sign that reads "STOP TANK CAR CONNECTED"?

After encountering a sign that reads "STOP TANK CAR CONNECTED," the appropriate action for the train crew is to ensure that cars must not be coupled to or moved without prior notification to the person who placed the sign. This requirement is crucial for safety, as the sign indicates that there are tank cars connected that may contain hazardous materials, and any movement could disturb a potentially dangerous situation.

Maintaining communication with the individual who placed the sign allows the crew to receive vital information about the status of the tank cars and the nature of the materials they contain. This precaution minimizes risks such as spills, leaks, or accidents that could occur if the cars are moved without understanding the context of the situation being signaled by the sign.

The other responses do not adequately address the safety protocols that need to be observed regarding connected tank cars. Moving all cars away immediately or only moving empty tank cars does not take into account the necessary communication and confirmation needed to ensure safe operations. Notifying a supervisor and then proceeding with movement also fails to prioritize direct communication with the person responsible for the sign, which is critical in handling situations involving tank cars.