List one common type of rail defect.

Rail cracking is a common type of rail defect that can significantly impact the safety and performance of rail systems. It typically occurs due to various stress factors, such as fatigue from the repeated loading of trains, environmental conditions, or manufacturing flaws. Cracks can propagate over time if not detected and addressed, potentially leading to failure of the rail and posing serious safety hazards for trains and their passengers. Regular inspections and maintenance are critical to identify such defects early and prevent catastrophic incidents. Monitoring for rail cracking involves techniques such as ultrasonic testing, which helps ensure the integrity of the rail infrastructure.

In contrast, while track displacement can affect alignment and stability, and rail vibration is related to the dynamic behavior of trains running on the rail, these issues are not classified as defects in the rail itself but rather conditions that can arise from other factors. Surface rust, while a concern for rail lifespan and maintenance, does not structurally compromise the rail in the same way that cracking can.