The paper describes method to evaluate residual service life of reinforced concrete structures of railroad bridges, enabling to consider the presence of defects, operation history, and climatic factor to calculate probabilities of the structural state at any given moment of time. The model based on semi-Markov process was used to predict the state and calculate reliability indicators of structures and their elements. The data sources for reliability models are results of observations of the structural state, mathematical models of occurrence and development of defects. The example of determining residual service life of reinforced concrete superstructure is given based on the mathematical model of working reinforcement corrosion development.