Under which circumstance must a "Light Engine Running Air Brake Test" be performed?

A "Light Engine Running Air Brake Test" must be performed when taking charge of an engine that is not coupled to other equipment. This situation arises because the engineer is responsible for ensuring that the air brake system is functioning properly before the locomotive is put into service. Since no other cars are present to test the connections, it becomes essential to verify the integrity and readiness of the engine's braking system independently.

Conducting this test under such circumstances ensures safety and operational readiness. It allows for the assessment of any potential issues with the direct air brake system of the engine, which needs to be confirmed prior to use, especially in operations that could involve significant forces or speeds later on.

The other options present scenarios that do not specifically require a Light Engine Running Air Brake Test. For instance, losing communication with the dispatcher may necessitate other procedures, but it does not automatically require this specific air brake test. Similarly, adverse weather conditions and regular distance intervals like 500 miles have their own safety protocols and do not necessitate this particular type of test. Thus, the requirement to perform the test specifically pertains to the situation of taking charge of an uncoupled engine.