In the event of an internal failure of the onboard apparatus, what must an engineer do?

When faced with an internal failure of the onboard apparatus, the protocol dictates that the engineer must cut out the Automatic Cab Signal Enforcement System (ACSES). This action is critical as it ensures the safety and proper control of the train in response to the failure of onboard systems. ACSES is designed to enforce signals and ensure safe operation; if there's an internal malfunction, maintaining its function could pose significant risks, including potential safety breaches.

In addition, cutting out the system allows the engineer to maintain control and make necessary decisions without relying on compromised equipment. This is a standard safety measure that helps prevent further complications that could arise from a failure.

While addressing issues with personnel, such as contacting a maintenance team or informing passengers, becomes relevant later in the situation, the immediate and necessary step involves ensuring the train can operate safely without the failed apparatus.