Which defect can be identified in the brake system of a single locomotive?

Identifying a defective brake shoe is crucial because it directly impacts the locomotive's ability to safely stop. A brake shoe is a component that makes contact with the wheel to generate the friction necessary for braking. When a brake shoe is worn or damaged, it can lead to reduced braking efficiency, which poses a safety risk not only to the locomotive but also to any trains or vehicles nearby. Routine inspections involve checking brake shoes for wear, cracks, or other signs of damage, making this defect one that is detectable and requires immediate attention to maintain safe operation.

In contrast, while the inoperative sander and operational brake rigging are important aspects of a locomotive's performance, they do not represent a direct defect in the braking system itself. The functionality of the air brake system can be assessed as part of broader technical checks, but it does not specifically identify a defect like the condition of a brake shoe does. Thus, the focus on the brake shoe highlights a specific, actionable component that directly affects braking capability and safety.