Which of the following best describes 'block limit signals'?

Block limit signals refer to signals that are specifically designed to control the entry and exit of trains within designated blocks of track. These signals are crucial for maintaining safe train operations by ensuring that only one train occupies a block at any given time. This control helps prevent collisions and ensures that trains can safely enter or exit blocks as needed.

The function of block limit signals primarily pertains to train movements, providing clear indications to conductors and engineers about when they can proceed into a block and when they must stop. By regulating train movements within specific segments of track, these signals play a vital role in maintaining overall system safety and efficiency.

Other choices suggest different uses for signals that do not align with the primary purpose of block limit signals. For example, there are signals specifically for freight operations, visual cues for passengers which serve a completely different function, and automated systems that rely on GPS, none of which accurately describe the role of block limit signals in train operations.