The introductory part of the thesis describes the development of bridges and the emergence of steel bridges in Slovenia. It discusses the proportion of steel bridges in Slovenia, their type of construction, static system and type of intersection. The second part of the thesis discusses the requirements given in the rules on the substructure of bridging structures and provides a detailed overview of individual elements (track, clear profile, ground plan geometry, distance between track centres, sleepers, jointed track, switch, insulation joint, load testing, signs on constructions). As for the third part, it contains a summary of Eurocode 3, the process of designing a steel construction as well as loads and load cases. It also points to the significance of structural details for the construction's life span. Moreover, it touches on the issues of steel bridges – corrosion, descriptions and types of corrosion, preparation of steel surfaces for anti-corrosion protection, and approaches to tackling corrosion. The majority of steel railway bridges in Slovenia are over 80 years old and in a very bad condition in terms of the level of corrosion. The situation is a result of infrequent maintenance of the anti-corrosion protection and even neglect of this aspect of maintenance. To this end, the final part of the thesis discusses how corrosion causes fatigue and decreases pylon thickness, and outlines the methods for determining the loss of material due to corrosion. Considering the fact that the majority of the bridges have already exceeded their envisaged life span and that their maintenance was rather poor, it is clear that they are worn out and in need of thorough renovation.