Abstract: The key factors to be evaluated to ascertain the remaining safe fatigue life of a railroad bridge are (a) geometric structural fatigue detail category and its stress range to cycle capacity (S – N) and (b) the applicable stress ranges. Laboratory experimentation has defined the geometric structural fatigue detail categories and their stress range to cycle capacities (S – N) at constant amplitude to a level that is generally much better defined than the applicable stress range. The applicable stress ranges depend on the load spectra and the geometric properties of the structure being evaluated. Calculated stress ranges are rarely accurate enough for realistic remaining safe fatigue life predictions. Critical stress ranges are generally quite low in relation to the ultimate stress capacity for which most analytical models have been developed, and field strain gauge testing, assuming the cost is worth the saving, is often necessary to get a more reasonable estimate of stress range to applied load. The next and often most difficult factor to evaluate is the appropriate load spectra, particularly for structures already subject to many years of loading. The present paper will discuss the issues raised above and show a number of short examples of the influence of these various factors.