What does a defective center sill include?

A defective center sill refers to structural integrity issues that can compromise the safety and functionality of rail cars. In this context, the criteria for identifying a defective center sill include specific measurements of cracks and bends that indicate significant structural failure.

The correct choice highlights that a center sill is considered defective if it has cracks exceeding 6 inches and is permanently bent more than 2 1/2 inches in a 6-foot area. These measurements are critical because they represent severe structural damage that could lead to failure during operation, particularly when the rail car is subjected to stress loads. A crack of over 6 inches indicates a significant risk of fracture, while a permanent bend of over 2 1/2 inches suggests deformation that could affect the alignment and performance of the rail car.

In contrast, the other options either present measurements that do not meet the severity required to categorize the center sill as defective or involve conditions (like rust) that may not structurally compromise the center sill to the extent of the described situation. Understanding these specifications is essential for rail mechanics to ensure safe operation and maintenance of rail vehicles.