Electrified railway systems are the current focus of energy efficiency studies due to the increasing constraints of environmental sustainability policies. Railway System Operators (RSOs), also encouraged by market deregulation, are thus examining how to update infrastructures and vehicles and/or apply new operating strategies. Given the intrinsic relation between railway systems and their primary feeding grid, each measure applied to a railway system impacts directly on the power grid which supplies the system. Against this background, this paper aims to ascertain the impact of an energy efficiency strategy applied by the RSO system on its primary grid. The energy-efficient strategy is identified as to preserve timetable stability and service quality in a passenger-oriented perspective. Moreover, a method to model the interaction between the systems in terms of power flow exchanges is proposed. The model is used to verify how an eco-driving technique applied to a metro system affects the state variables of the distribution grid. In particular, the primary grid, the traction power substations and the railway system are modelled according to real data. The case study is the Metro Line 1 operating in the city of Naples (Italy). The numerical simulations are carried out with a two-step procedure: first, the power requirements of the rolling stock as a function of the imposed operating service are calculated by OpenTrack ® software; the state variables of the distribution grid are then evaluated by means of the power flow approach.