PPMs utilise a rotating flywheel as a store of kinetic energy which is then used to power the vehicle. A typical PPM flywheel is made from steel laminates, approximately in diameter and in mass, designed to rotate at a maximum speed of 2,500 rpm. The flywheel is mounted horizontally at the centre of the unit, beneath the seating area. The flywheel is driven by an internal combustion engine or an electric motor. The flywheel is connected to the rail wheels via a hydrostatic variable transmission system. The wheels are driven without conversion into electricity as many other railcars utilizing flywheel energy storages do.