What visibility requirement does the FRA stipulate for a derail?

The Federal Railroad Administration (FRA) stipulates that a derail must be clearly visible to ensure safe operations. This visibility requirement is critical for identifying the location of the derail, which is a safety device used to prevent a train from inadvertently moving onto a segment of track that is not in service or is not safe for travel.

Clear visibility aids in preventing accidents by allowing train operators, engineers, and maintenance personnel to spot the derail easily, especially in low-light conditions or during inclement weather. The design and placement of derails are intended to enhance safety by minimizing the risk of them being overlooked, which could lead to dangerous situations where trains could derail unintentionally.

While other options suggest various safety features such as illumination or reflective markings, the primary requirement is that the derail must be distinctly seen to fulfill its function effectively. These additional features can enhance visibility but are not explicitly mandated as the key requirement by the FRA, thus supporting the correct understanding that clear visibility is paramount.