What indicates a Main Reservoir rupture on a PA4 train?

The correct indication of a Main Reservoir rupture on a PA4 train is a very slow loss of Brake Pipe pressure. This phenomenon occurs because when the Main Reservoir experiences a rupture, the primary air supply pressure can escape gradually, affecting the Brake Pipe pressure more slowly rather than instantaneously.

The Brake Pipe operates under a distinct pressure that is crucial for the functioning of the train's braking system. When a rupture in the Main Reservoir happens, the air gradually leaks out, which leads to a slower, more sustained decrease in air pressure in the Brake Pipe. This slow loss can indicate that the reservoir is compromised.

In contrast, other options describe scenarios that would not align with the dynamics of a Main Reservoir rupture. An immediate loss of all air pressure would suggest a catastrophic failure rather than a rupture, while a rapid increase in Straight Air pressure is indicative of a different issue, possibly related to a brake cylinder malfunction. Lastly, noise from the compressor may indicate a functioning system or maintenance issue but does not directly correlate with the rupture of the Main Reservoir. Each of these scenarios highlights different operational issues, reinforcing why the gradual loss of Brake Pipe pressure specifically points to a Main Reservoir rupture.