What action must a train operator take if there is an emergency and customers are allowed to leave?

In an emergency situation where customers are allowed to leave, opening the emergency brake valve is the correct action for a train operator to take. This step ensures that the train can come to a safe stop or prevent it from moving further, allowing for the safe evacuation of passengers.

The emergency brake valve is designed to quickly slow down or stop the train in critical situations, ensuring that passengers can disembark without the risk of injury that might occur if the train were to continue moving. By using this mechanism effectively, a train operator prioritizes the safety of all individuals on board.

Engaging the emergency brake also helps to control the situation, enabling the operator to coordinate with emergency services and facilitate the safe exit of passengers. It's crucial in an emergency not only to stop the train but also to maintain a secure environment for those leaving.

In contrast, the other options listed would not promote the safety or well-being of passengers. Deactivating safety systems or starting to move the train slowly contradicts the protocol for ensuring passenger safety. Calling for emergency services is essential but should follow securing the train and ensuring passengers are evacuated safely.