Under what condition is the cab signal apparatus considered to have failed?

The cab signal apparatus is considered to have failed under the condition described in the correct option, which states that the cab signal changes twice in a row without conforming. This situation indicates a malfunction because the cab signal system is designed to provide accurate and reliable information to the train operator regarding signal aspects. When the cab signal changes twice consecutively but does not conform to expected behavior or indications, it creates a heightened risk for the safe operation of the train, as the engineer may receive conflicting or invalid information regarding the upcoming signals.

This failure requires immediate action to ensure safety, as the system is integral to maintaining appropriate speed and awareness of signal indications en route. In comparison, other options such as the first signal failure, an excessive train speed, or loss of communication with the dispatcher do not specifically indicate a failure of the cab signal apparatus itself. Such conditions may affect train operations but do not directly indicate a malfunction of the cab signal processing or indication functionality. Therefore, the scenario of non-conforming changes serves as a clear indicator of system failure.