What restriction applies to non-certified rail installed in a main line required to be tested by a rail detector?

The correct answer indicates that non-certified rail installed in a main line that requires testing by a rail detector must not exceed Class 2 speeds. This restriction is significant because non-certified rail typically lacks the same level of verification for structural integrity and safety as certified rail. Consequently, allowing such rail to remain in service at speeds exceeding Class 2 can increase the risk of derailments or other safety hazards. Class 2 speed limits are set to ensure a balance between operational efficiency and safety, recognizing the limitations and potential issues associated with non-certified materials.

In contrast, higher classes of speeds (like Class 3 or above) would put non-certified rail under greater strain and increase the likelihood of failure, which is why those options are not applicable. The notion that there are no speed restrictions does not hold, as safety regulations are designed to mitigate risks associated with infrastructure that does not meet stringent certification standards. Therefore, adhering to the Class 2 speed limit serves as a precaution to protect both the rolling stock and passengers from potential accidents.