A trackside energy storage system installed at a DC tram-trolleybus substation is considered from the standpoint of its ability to capture the braking energy of electric vehicles. The dependence of energy savings on the power capability and energy capacity of such a system is studied. For this purpose a method of stochastic modeling is applied. It uses diagrams of experimentally recorded power of vehicles running within the feeding zone of the substation. The method is verified by applying probabilistic approach that uses the power probability density functions obtained from the same power diagrams. The modeling was performed for one substation feeding tram line in the Riga city. Results of this work allow finding the optimum between energy storage system parameters and recovered braking energy at given conditions. The proposed method can be applied for energy storage system sizing not only for current situation, but also for future taking into account growth of transport fleet and changes in transport organization.