Each railroad covered by 49 CFR 228.407 must perform an analysis of the work schedules of its train employees who are engaged in commuter or intercity rail passenger transportation and identify those schedules that, if worked by such a train employee, may be at risk for a level of fatigue at which safety may be compromised. A level of fatigue at which safety may be compromised, the fatigue threshold, shall be determined by procedures that use a scientifically valid, biomathematical model of human performance and fatigue. This document describes the criteria and procedures that the Federal Railroad Administration (FRA) will use for certifying the scientific validity of a biomathematical model and for determining a fatigue threshold in a biomathematical model. This document also identifies specific such models that are acceptable. Furthermore, it describes the procedures that must be used both to analyze work schedules and to report FRA those schedules that are at risk for exceeding the fatigue threshold.