What device initiates an emergency application of the air brakes if a train passes a fixed signal showing red?

The correct answer is that the device that initiates an emergency application of the air brakes when a train passes a fixed signal showing red is the automatic stop. This system is designed to enhance safety by ensuring that trains adhere to signals that indicate danger or require a stop. When the train approaches a signal displaying red, the automatic stop mechanism detects this violation and automatically applies the brakes, bringing the train to a halt to prevent any potential accidents.

The automatic stop system operates independently of the train operator's manual actions, allowing for a quick and efficient response to potentially dangerous situations. It is a critical safety feature, particularly in avoiding collisions or derailments due to disregarded signal indications.

In contrast, while manual brakes and emergency brake switches can be used by train operators to stop the train, they rely on the operator's actions and responses, which may not always be immediate in emergency situations. An alert signal does not apply the brakes directly; it serves to notify operators of the signal status but does not take action on its own.