What color signal indicates that a train operator must slow down?

The signal that indicates a train operator must slow down is yellow. This color serves as a warning signal, indicating that the train is approaching a condition that may require reduced speed or caution. Yellow signals may inform operators of various situations, such as the need to prepare for a stop signal or an upcoming change in track conditions.

In contrast, red signals typically indicate that a train must stop, while green generally signifies that it is safe to proceed at normal speed. Blue signals usually pertain to work equipment and do not indicate a change in speed for the train. Therefore, understanding the meanings behind these color signals is crucial for safe train operation and adherence to operational protocols.