What happens if the main reservoir pressure drops below 95 psi?

When the pressure in the main reservoir drops below 95 psi, it triggers the application of the parking brake. This mechanism is designed to enhance safety by ensuring that the train does not operate under conditions where there is insufficient air pressure to effectively control the braking systems. At pressures below this threshold, the braking system may become ineffective, increasing the risk of accidents.

Applying the parking brake ensures that the train remains stationary, preventing any unintended movement due to a failure in the air brake system. This is critical for maintaining safety standards and operational integrity, as it allows prompt action to be taken to address the pressure issue before resuming operation.

The other options do not accurately represent the consequences of a drop below this pressure level. For instance, the train stopping completely or accelerating is not typical behavior in this scenario, nor is it standard for the engineer to receive a warning signal under these specific conditions. Instead, the automatic application of the parking brake serves as a proactive safety measure.