What should a train engineer do when leaving CBTC mainline territory?

When leaving CBTC (Communications-Based Train Control) mainline territory, the train engineer should acknowledge the transition and then operate normally according to the rules and regulations of the territory they are entering. This is crucial because different territories may have distinct operational protocols, signaling systems, and speed limits that need to be respected.

By acknowledging the territory and operating normally for the next section, the engineer ensures a smooth and safe transition between different control systems. This practice helps maintain safety and efficiency in train operations, as each area may have specific requirements that differ from CBTC operations.

Other options suggest immediate actions that may not align with standard operating procedures or could potentially cause delays or safety concerns. For instance, switching to manual operation or stopping immediately could interrupt service and is typically not necessary unless a specific safety situation arises.