Which chord length is exclusively used on class 1 and 2 curved tracks?

The correct choice indicates that the 62-foot chord is exclusively used on class 1 and 2 curved tracks. This specification is tied to the design and safety standards required for railways to ensure the smooth navigation of trains along curves. The 62-foot chord length provides an adequate measure for assessing the track's alignment and curvature, which is crucial for maintaining stability and minimizing the risk of derailments.

On the other hand, the 31-foot chord may be utilized in certain contexts, but it is not the exclusive standard for class 1 and 2 tracks. Additionally, the option suggesting that any length chord is acceptable does not align with industry regulations, which mandate specific chord lengths based on track classifications to ensure safety and efficiency. Thus, the choice of the 62-foot chord reflects adherence to those safety standards and engineering best practices for curved tracks in these classes.