Which device is necessary on the controlling locomotive to qualify a train's air brake system using AFM?

The maintaining-type automatic brake valve is essential for qualifying a train's air brake system using Automatic Function Monitoring (AFM). This is because the maintaining-type valve plays a crucial role in the operation of the air brake system, allowing for the continuous adjustment and maintenance of brake pipe pressure while the train is in operation.

The AFM system relies on accurate and consistent pressure readings to ensure that the air brake system is functioning correctly. The maintaining-type automatic brake valve sustains the required brake pipe pressure that is necessary for the proper application and release of the brakes. Without this type of valve, the AFM would not be able to effectively monitor or ensure the integrity of the train's braking system, leading to safety concerns.

Other devices mentioned, such as an automatic brake control panel or a manual brake release switch, do not specifically ensure the ongoing maintenance of air pressure needed for qualifying the brake system. While useful, they do not fulfill the critical functionality of maintaining brake pipe pressure required for successful AFM qualification. A pressure gauge with an audible alarm, though important for alerting the crew to pressure changes, does not actively maintain the pressure, so it does not replace the necessity of the maintaining-type automatic brake valve in this context.