What is the main function of the air brake system on a locomotive?

The main function of the air brake system on a locomotive is to slow down or stop the train safely. The air brake system is designed to use compressed air to apply pressure to brake shoes, which then create friction against the train's wheels. This process enables the train to decelerate smoothly and come to a complete stop when necessary.

Safety is a critical aspect of train operation, and the air brake system is engineered to provide reliable stopping power under various conditions, ensuring the safety of the crew, passengers, and cargo. It is essential for managing the massive momentum of a train effectively, especially when approaching stations, signal changes, or during emergency situations.

The other options involve functions that are not directly related to the air brake system. For example, controlling engine performance pertains to engine management systems, while coupling and uncoupling cars is a separate operational task that involves other equipment and procedures. Monitoring fuel consumption is typically handled by different systems that focus on tracking the fuel efficiency of a locomotive rather than its braking capabilities.