The strength limit state concerns the axial, bending, shear, and torsional resistance of members to tensile yielding, tensile fracture, and/or compressive instability. Fatigue, or the failure of steel at nominal cyclical stresses lower than yield stress, is a phenomenon that occurs due to the fluctuating nature of railway traffic and the presence of stress concentrations in the superstructure. Railway live loads are a longitudinal series of moving concentrated axle or wheel masses or loads that are fixed with respect to lateral position. The static analysis of railway superstructures involves the determination of the deformations and stresses in members caused by the moving loads. The criteria for maximum shear force and bending moment in a simply supported span illustrate that loads may be stepped across the span and their effects investigated at the location of interest. Influence lines of a superstructure element illustrate the variation in the effect at the location as a unit load traverses the superstructure.