The article presents calculations enabling the selection of an on-board energy storage device to replace diesel-powered trains with modern storage multiple units on the example railway line Gdynia Główna - Hel. The pioneering and currently used storage electric multiple units, mainly in Europe, were presented. The condensed results of the inventory of the line in question, including the technical condition of the stations of the non-electrified line 213 Reda - Hel, are included. Moreover, an analysis of the terrain possibilities of building new passing loops aimed at increasing the capacity of line 213 is presented. The calculations of the theoretical passage and energy demand of the storage train unit on the non-electrified part of the analyzed route are presented. The results of the calculations were used to select the on-board energy storage consisting of batteries and supercapacitors.