Electrified railway systems are stimulating worldwide a growing interest in innovative models, technologies and operating measures to increase energy efficiency and sustainable development, while satisfying customer requirements. Railway System Operators (RSOs) are widely studying how technologically renewing infrastructures and vehicles or applying eco-driving techniques. Any measure applied to railway systems impacts also on the primary grid, which supplies the system in terms of power and energy. Modes of impact do not depend only on the type of the measure, but also on the technological characteristics of the railway system and its modalities of operating. Aiming at verifying the interaction between a real metro system and its primary distribution grid, this paper proposes a methodology to model the interaction between the systems in terms of power flow exchanges. The model is used to verify how eco-driving techniques applied on the railway system can affect the main state variables of the primary grid, modifying the operating values of the instantaneous power and net energy. In this paper, primary grid, traction substations and railway system are modelled according to real data. The observed case study is the line 1 metro system operating in the city of Naples. Numerical simulations are carried out by means of a two-step procedure: 1) the OpenTrack software is used to determine the power requirements of the railway system as a function of the imposed operating constraints; 2) the power flow approach assesses the operating state of the distribution grid.