What temperature threshold requires a speed reduction of 10 MPH below the maximum authorized timetable speed?

A temperature threshold of 85 degrees Fahrenheit requires a speed reduction of 10 MPH below the maximum authorized timetable speed. This rule is put in place to ensure safety and train performance, as higher temperatures can affect the integrity of the tracks and the functioning of the trains. When temperatures rise, it can lead to track expansion or other issues that may disrupt safe operations. By implementing a speed reduction, the possibility of accidents is minimized, and the trains can operate more reliably under varying weather conditions.

In this scenario, the other temperature options either do not trigger this specific safety requirement or are too low to warrant such a speed adjustment, making them less relevant for this situation.