What is a primary function of the duplex air gauge?

The primary function of the duplex air gauge is to check air pressure. This gauge provides a visual representation of the air pressure within the train's pneumatic systems, which is crucial for the operation of airbrakes and other air-operated devices. Monitoring air pressure helps ensure that the braking systems have sufficient pressure to function correctly, which is essential for safe train operation. The accuracy of the air pressure readings enables train operators to make informed decisions about the train’s operational capability and safety.

The other functions mentioned in the choices are not the primary role of the duplex air gauge. While speed is monitored through speedometers, brake effectiveness is assessed through different testing protocols, and throttle control is managed through separate controls within the train's operating system. Thus, the focus of the duplex air gauge on air pressure is vital for safety and performance in train operations.