This paper focuses on three alternative railway systems (i.e., railway, urban metro and city tram). An approach to assess the size of an on-board energy storage unit is proposed. The unit is designed to supply the three trains along a catenary-free track. The equations of motion are formulated in a common case. The approach solves the system of equations by means of an optimization procedure where a set of constraints on the representative variables is assigned. Service duty cycle and energy requirements are characteristics of the three alternative systems. The results of the numerical simulations focus on the design of a battery-based storage unit to be equipped in the rolling stock vehicles. Specifically, the performance required to the storage units in the three alternative configurations are investigated and compared. Some final notes pointed out the impact of the design constraints on the unit sizing.