What action should the train engineer take when in the yellow zone speed dial?

When the train engineer is in the yellow zone speed dial, applying service braking to slow down the train is the correct course of action. The yellow zone typically indicates a cautionary state where the train is approaching the limit of safe speed or may encounter restrictive conditions ahead. This necessitates a proactive response to ensure safety by reducing speed, as it prepares the train for potential changes in track signals or conditions.

By selecting to apply service braking, the engineer mitigates risks associated with excessive speed in a cautionary situation. This action aligns with operational protocols aimed at prioritizing safety and readiness for any forthcoming adjustments required due to track or signal conditions.

Other responses, such as increasing throttle, checking systems for errors, or notifying passengers about a delay, would not appropriately address the immediate need to manage speed and ensure safety when in a cautionary zone.