What does Positive Train Control (PTC) apply brakes in response to?

Positive Train Control (PTC) is a sophisticated safety system designed to automatically monitor and control train movements to enhance safety on the rail network. The primary function of PTC is to apply the brakes in response to any signal or condition that requires a train to stop. This includes not just signals indicating the need to halt but also various other scenarios such as speed restrictions, track conditions, and other operational hazards.

When the system detects a situation where a stop is required, it instantly responds to ensure that the train slows down or halts, thereby preventing potential accidents and ensuring adherence to operational protocols. This functionality is critical for maintaining safe distances between trains, preventing train-to-train collisions, and managing hazardous situations effectively.

Other options suggest limited scenarios for braking, which do not encompass the comprehensive nature of PTC's capabilities. For instance, considering only temporary speed restrictions or relying on passenger input would not guarantee the wide-reaching safety measures that PTC is designed to provide.