What happens when an interlocking appliance is secured?

When an interlocking appliance is secured, it means that it is in a state where it is electronically disabled and mechanically locked. This process is crucial for maintaining safety and ensuring that the signals and switches within the interlocking system cannot be inadvertently changed or manipulated. By locking the interlocking, it prevents any unauthorized access or operational errors that could lead to unsafe train movements.

The mechanism involves specific protocols designed to ensure the reliability and safety of train operations, particularly in complex junctions or areas where multiple tracks converge. In this secured state, train movements are tightly controlled, allowing only those that are predetermined and safe, which upholds the integrity of the railway system.

The other options do not accurately reflect the state of a secured interlocking appliance. Unrestricted train movements would imply no controls are in place, which contradicts the purpose of securing the interlocking. Operating solely by manual control does not apply, as the securing process typically involves electronic components that are inherently disabled. Lastly, a secured state does not indicate an emergency; rather, it is a standard operational procedure to ensure safety.