When may the engineer increase the speed of the train after receiving a more favorable cab signal indication?

The engineer may increase the speed of the train immediately after receiving a more favorable cab signal indication because such an indication signifies that the conditions ahead have improved, allowing for a higher permissible speed. The cab signal system is designed to provide real-time updates regarding the train's operating conditions, enhancing safety and ensuring adherence to speed regulations.

Once the more favorable indication is displayed, there are no additional requirements for the engineer to confirm or wait; they can respond right away to optimize the train's speed, provided they remain aware of the surrounding environment and train dynamics. This immediate action helps maintain operational efficiency and align with the goal of timely service delivery.

The other options would introduce unnecessary delays or conditions that are not stipulated by the operating rules, which emphasize responsiveness to signal changes in favor of safety and efficiency.