What is a critical action the train operator must take when encountering unknown brake applications?

When encountering unknown brake applications, securing the train is a critical action for the train operator. This step is essential because it ensures the safety of the train, its passengers, and any personnel. Securing the train prevents any unintended movement and provides a stable situation from which to assess and address the brake issue.

In the event of an unknown brake application, the operator must prioritize safety and control of the train. By securing the train, the operator can then proceed to investigate the source of the brake application, determine if there is a malfunction, and plan the next steps with a clear understanding of the situation.

The other options may seem relevant, but they do not adequately address the critical need for immediate safety measures. Inspecting the brakes immediately might take place after securing the train, as does notifying passengers. Continuing to the next station would pose significant risks without understanding the cause of the brake application. Thus, ensuring the train is secure first provides the necessary foundation for further actions.