When does the air compressor typically operate?

The air compressor operates based on the pressure in the main reservoir and is governed by a pressure switch or governor. When the air pressure in the main reservoir drops below the predetermined setting of the governor, the compressor will activate to replenish the air supply. This automatic operation ensures that the air brake system remains fully functional by maintaining adequate air pressure for braking and other pneumatic systems on the train.

In contrast, the other choices do not accurately describe the operational behavior of the air compressor. For example, while the train is moving, the air compressor might or might not be actively running depending on the air pressure status. Similarly, stating that it operates only at scheduled intervals ignores the need for real-time management of air pressure. The assertion that it activates only when the train is stopped is also incorrect, as the compressor can operate while the train is in motion if the air pressure in the reservoir falls below the set threshold.