An energy storage system comprising: a rail system having upper and lower storage yards (10, 12) with interconnecting track (14, 16) connecting the upper and lower yards (10, 12) and a plurality of track routing configuration control elements; a plurality of shuttle units supported by wheels and operably connected to an electrical motor and an electrical generator, said shuttle units further having a support structure and integral transfer mechanism (80, 86, 87) for removably carrying masses; an electrical interconnection between the motors and generators and a utility electrical grid; a control system in communication with the utility electrical grid, the plurality of shuttle units and the rail system control elements and said control system executing a first control sequence responsive to requirements of the utility electrical grid under conditions of excess power to store energy and executing a second control sequence responsive to requirements of the utility electrical grid under conditions of required additional power for providing power to the utility electrical grid, said first control sequence causing selected shuttle units to retrieve masses located in the lower storage yard and using the operably connected motor to drive the selected shuttle units between the lower storage yard and the upper storage yard, said control elements configured to route the selected shuttle units, and causing the selected shuttle units to offload the masses in the upper storage yard to store energy; said second control sequence causing selected shuttle units to retrieve masses located in the upper storage yard and using the operably connected generator supplying power to the utility electrical grid by selective regenerative braking of the selected shuttle units between the upper storage yard and the lower storage yard to provide the required power, said control elements configured to route the selected shuttle units, and causing the selected shuttle units to offload the masses in the lower storage yard; a rate of input for storing excess power and a rate of output for providing required power varied by the control system by controlling speed or quantity of selected electric powered shuttle units. a rail system having upper and lower storage yards (10, 12) with interconnecting track (14, 16) connecting the upper and lower yards (10, 12) and a plurality of track routing configuration control elements; a plurality of shuttle units supported by wheels and operably connected to an electrical motor and an electrical generator, said shuttle units further having a support structure and integral transfer mechanism (80, 86, 87) for removably carrying masses; an electrical interconnection between the motors and generators and a utility electrical grid; a control system in communication with the utility electrical grid, the plurality of shuttle units and the rail system control elements and said control system executing a first control sequence responsive to requirements of the utility electrical grid under conditions of excess power to store energy and executing a second control sequence responsive to requirements of the utility electrical grid under conditions of required additional power for providing power to the utility electrical grid, said first control sequence causing selected shuttle units to retrieve masses located in the lower storage yard and using the operably connected motor to drive the selected shuttle units between the lower storage yard and the upper storage yard, said control elements configured to route the selected shuttle units, and causing the selected shuttle units to offload the masses in the upper storage yard to store energy; said second control sequence causing selected shuttle units to retrieve masses located in the upper storage yard and using the operably connected generator supplying power to the utility electrical grid by selective regenerative braking of the selected shuttle units between the upper storage yard and the lower storage yard to provide the required power, said control elements configured to route the selected shuttle units, and causing the selected shuttle units to offload the masses in the lower storage yard; a rate of input for storing excess power and a rate of output for providing required power varied by the control system by controlling speed or quantity of selected electric powered shuttle units.