What situation necessitates a Class 3 air brake test?

A Class 3 air brake test is required specifically after a caboose change because this situation involves a significant alteration in the makeup of the train. When a caboose is replaced, it is essential to ensure that the air brake system is functioning properly throughout the entire train. This test verifies that the brakes will function effectively in the new configuration, which is critical for safety during operation.

In contrast, the other scenarios provided do not inherently necessitate a Class 3 air brake test. For instance, having an empty train does not automatically require this test unless other conditions dictate it. Similarly, reclassifying freight cars may involve modifications, but it does not specifically trigger the need for a Class 3 test unless a caboose change occurred concurrently. Lastly, while train stops require attention to safety and braking functionality, they do not automatically mean a Class 3 air brake test is necessary in every instance. Thus, the requirement to conduct this test after a caboose change stands out as a specific safety protocol that must be followed.