What happens if the MR pressure drops below 95 psi?

When the main reservoir (MR) pressure drops below 95 psi, the parking brake will apply as a safety measure to prevent potential accidents. This automatic activation helps to ensure that the train remains stationary and minimizes the risk of unintended movement due to insufficient air pressure for braking systems. Maintaining adequate air pressure is critical for operational safety, as it is necessary for various braking functions. If the pressure is below a certain threshold, activating the parking brake serves as a precautionary response to protect both the crew and the integrity of the train system.

The other choices, while they may pertain to different scenarios or systems, do not correctly describe the immediate action taken when MR pressure falls below the specified level. Thus, the application of the parking brake provides a direct and effective response to the situation at hand.