Why would a Light Engine Running Air Brake Test be necessary?

A Light Engine Running Air Brake Test is necessary when the controlling ends of a train have been changed. This situation occurs when the lead engine is switched or when the configuration of the train changes in a way that affects its braking system. Since the air brake system relies on specific interfaces and configurations to operate effectively, any changes in the ends can introduce variables that might affect braking performance. Conducting this test ensures that all components are functioning correctly and that the train can stop safely, which is crucial for maintaining safety standards.

In contrast, while weather conditions might impact operations, they do not directly necessitate a Light Engine Running Air Brake Test unless related to a change in the controlling ends. The dispatcher’s request does not inherently determine the need for testing unless it pertains to safety protocols specific to the situation. Lastly, the distance traveled, such as less than 10 miles, does not inherently justify or negate the need for performing the test; rather, it's about the safety and integrity of the brake system following any changes to train control.