In what state is an interlocking appliance when it has been secured?

The correct understanding of an interlocking appliance being "secured" refers to it being disabled and locked in the required position. In railway operations, interlocking systems are critical for ensuring safety by preventing conflicting movements in critical areas, such as track junctions or crossings.

When an interlocking appliance is secured, it means that it has been properly set to ensure that all movements of trains are safe and reliable and that no unintentional unlock can occur. This locking mechanism ensures that the appliance is not operational and cannot be overridden, which helps in maintaining a high level of safety on the tracks.

To contrast with the other options, being operational and manually overridable would imply that the appliance can still be manipulated in a way that could compromise safety. Active inspection implies that the appliance is subject to worker activity, which contradicts the definition of being "secured." Standby mode suggests some level of readiness for operation, which does not align with the concept of being locked in a position for safety reasons. Therefore, the focus on being disabled and locked in the required position is essential for understanding the operational safety protocols in locomotive engineering.