What signifies a main reservoir rupture on a PA5 train?

A main reservoir rupture on a PA5 train is indicated by a rapid loss of main reservoir pressure. This is because a rupture results in a significant and immediate drop in pressure due to the sudden escape of air from the reservoir, which is designed to store compressed air. As air escapes, the pressure in the reservoir falls quickly, making this a key indicator for identifying a rupture.

The other choices may indicate issues related to air systems but are not specifically indicative of a reservoir rupture. For instance, a sudden increase in pressure may suggest an obstruction or a malfunction rather than a rupture. The complete stop of air compressors could be a symptom of various issues, including power loss or operational settings, and does not directly correlate with the condition of the main reservoir. An increase in MR pressure above 95 psi typically indicates normal system operation rather than a rupture; thus, it is not associated with the scenario of a rupture occurring.