What should a train engineer do in response to a CBTC TOD failure while in motion?

When a train engineer experiences a CBTC (Communications-Based Train Control) TOD (Train Operations Display) failure while the train is in motion, the appropriate response is to operate the TOD selector to "Backup" and notify the Trainmaster. This action is crucial because switching to "Backup" mode allows the train to operate safely under alternative control parameters, ensuring that the train can continue its journey with minimal disruption while maintaining safety protocols.

Operating the TOD in "Backup" mode helps in regaining some level of operational control despite the failure. Notifying the Trainmaster is also essential as it allows for coordination of the situation and ensures that necessary support and resources can be mobilized in case of further complications. This response is systematic and aligns with established protocols for handling such failures.

The focus on maintaining operational capability while also ensuring communication with higher authorities reflects the priority of safety and efficiency in train operations. This response contrasts with other options, which might lead to unnecessary alarm, complications, or disruption of service.