For which scenario is the Intermediate Air Brake Test (Class 1A) required?

The Intermediate Air Brake Test (Class 1A) is specifically required for a unit train that has traveled a distance of 1,000 miles. This requirement is in place to ensure that the braking system is functioning properly after the train has covered significant distances. The check helps maintain safety standards by verifying that all components of the air brake system are in good working condition, thus reducing the risk of brake failures that could lead to accidents.

This process reflects the importance of regular maintenance and inspections, especially for unit trains that often consist of long chains of cars, requiring more careful monitoring of systems that affect their operation. While other options involve time or distance measures not applicable to the Intermediate Air Brake Test, the specific focus on the 1,000-mile interval underscores the need for diligence in management of train safety and reliability in operations.