What does the term 'holding pressure' refer to in air brakes?

The term 'holding pressure' specifically refers to the pressure that is maintained in the brake cylinders when the brakes are applied. This pressure is crucial for ensuring that the brakes remain engaged, effectively allowing the train to slow down or come to a stop as needed. When the air pressure is appropriately maintained in the brake cylinders, it keeps the brake shoes pressed against the wheels, thereby providing the necessary braking force.

In contrast to this concept, the other options relate to different aspects of train operations and air brake functionality. The pressure needed to operate the train's engine pertains to the power and propulsion systems, which are separate from the braking system. The pressure released during an emergency stop is a dynamic situation, where the system quickly depletes pressure to achieve rapid deceleration, rather than maintaining it. Lastly, the pressure measured in the air compressor involves the supply side of the air brake system, focusing on air generation rather than maintaining the pressure in the brake cylinders themselves. Thus, holding pressure is essential for the effective functioning of air brakes during operation.