Gravity loads comprising dead, live, and impact loads are the principal loads to be considered for steel railway bridge design. Superstructure dead load consists of the weight of the superstructure itself, track, deck, utilities, walkways, permanent formwork, snow, ice, and anticipated future dead loads. Railroad locomotives and equipment vary greatly with respect to weight, number of axles, and axle spacing. Actual loads may exceed design loads during the life of a bridge. Actual rail cars with 25" increase in individual axle loads are not uncommon due to flat wheels, rail and track conditions, overloading, and/or unbalanced loading. As equipment traverse the length of an influence line or span, cyclical variable-amplitude stress ranges are induced in the superstructure members carrying the live load. The variable-amplitude load and stress cycles from actual freight trains are typically irregular and require a method of reducing the complex spectra to simple cyclic loads and stresses.