What is a principal indication of a Brake Pipe rupture on a PA-4 train?

The principal indication of a Brake Pipe rupture on a PA-4 train is that air is blowing from the brake valve. When a rupture occurs in the brake pipe, it leads to a loss of pressure in the system. This drop in pressure causes the brake valve to react, allowing air to escape, which is a clear signal that there has been a compromise in the brake circuit.

The sound of air escaping is quite distinct and serves as a critical alert for train engineers to take immediate action. Recognizing this sound allows the engineer to diagnose a potential failure in the braking system, which is crucial for maintaining safety and operational integrity. Quick identification of a Brake Pipe rupture is essential, as it can impact the braking capabilities of the train, underscoring the importance of being aware of such audible cues while operating the train.

Other options may reflect conditions related to air pressures but do not indicate a direct response to a rupture, thus making them less relevant compared to the clear and immediate indication of air escaping from the brake valve.