The problem of optimally sizing hybrid energy storage systems (HESS) installed in electric railway systems, considering the effect of regenerative braking is studied in this paper. HESSs combine traditional batteries and newly developed ultracapacitors, taking advantage of the high energy capacity of batteries and of the flexibility and ability to capture high power density of ultracapacitors. A novel mixed integer linear programming formulation that includes the counting of battery cycles is presented. Some particularities of battery operation are included in the model, like the dependence of its performance on the number of cycles and the depth of discharge (DOD). Results are reported first for a illustrative case study, allowing us to perform sensitivity studies for several parameters; results are also reported and for a larger case, closely resembling the problem faced by railway energy systems planners and operators.Index Terms-Electric railway systems, hybrid energy storage sizing, mixed integer linear programming. NOMENCLATUREThis section describes the sets, parameters, and variables used in the paper. A. Sets Set of batteries.Time period set. AC/DC inverter set. DC/DC inverter set. B. ParametersMinimum battery unit (MBU) cost .Daily MBU cost for each battery .Minimum ultracapacitor unit (MUU) cost .