What is required under the base of the rail for classes 3-5 track according to the FRA?

The requirement for classes 3-5 track under FRA regulations emphasizes that metal objects causing concentrated loads are prohibited. This is significant because concentrated loads can create stress points in the track structure that may lead to increased wear, potential track deformation, or even catastrophic failures. The integrity of the rail and its foundation is vital for safe train operations, especially in classes 3-5 where the track must support trains with certain weight and speed criteria.

Allowing metal objects that can create these concentrated loads undermines the safety and stability of the track. Thus, prohibiting such objects is a critical measure to ensure that the rail infrastructure can maintain its structural integrity and provide a safe environment for rail services.

In contrast, other options suggest various standards for materials that might be used under the base of the rail, which do not align with the fundamental requirements aimed at preventing any potential track instability and ensuring support strength.