What action must a train operator take when encountering a yellow caution lantern?

When a train operator encounters a yellow caution lantern, the appropriate action is to bring the train speed down to 10 mph. This signal indicates that caution is required, rather than a complete stop, which is why maintaining a speed of 10 mph is essential for safely navigating potential hazards or upcoming conditions that could affect the train's operation.

Reducing speed to this level allows the train operator to maintain control while still being prepared to react if further signals or obstacles are encountered. This prudent approach reflects the need for vigilance and preparedness in the operation of trains, balancing the need for speed with the requirements of safety.