What is the purpose of an air brake system?

The purpose of an air brake system is fundamentally to slow down or stop a train by utilizing pressurized air to apply the brakes. This system works by allowing air pressure to engage the brake cylinders, which then push brake shoes against the wheels, creating friction that slows the train down or brings it to a complete stop. This is crucial for ensuring the safety and control of trains, especially given their size and weight.

The design of air brakes allows for efficient operation over long distances and across varied terrains, which is critical for trains that must frequently stop and start. Unlike other braking systems, air brakes can effectively serve multiple carriages at once, which is essential for long freight or passenger trains.

While other options might relate to train operations, they do not serve the primary function of initiating braking action. For example, lifting trains for maintenance, controlling passenger doors, or regulating speed on slopes are activities that involve different systems and mechanisms that are not focused solely on the function of stopping or slowing the train as the air brake system is designed to do.