Despite the available achievements of science and production technologies, there is an annual increase in the number of fractures of the cast side frames of freight wagon bogies in the network, often leading to train crashes and collisions. Therefore, it is considered very important to take into account all the significant factors affecting the reduction of the resource of side frames in operation at the design stage of new structures. The aim of the work is to develop and scientifically substantiate recommendations for increasing the fatigue strength of the side frames of freight wagon bogies in operation on the basis of a refined methodology for assessing the strength of side frames, taking into account the presence of internal casting defects in dangerous sections. A method has been developed for calculating the side frames for strength, taking into account technological factors: the presence of internal casting defects in dangerous sections, mechanical properties reduced by 20% in the part, minimum wall thicknesses of the casting according to the drawing; degradation of mechanical properties for 30 years or more.