With the growing demand for passenger transportation, several requirements are needed to promote an efficient and reliable transportation system to the user. For these aspects, the rail transport is a major reference across the world. Even though it had loosen its strength in Brazil in the middle of the last century, it s currently shifting back to its real potential as it can be observed with the expansions in the Subway and in the Trains of the Rio de Janeiro, São Paulo and others major capitals. The present paper considers intrinsic structures and characteristics of the networks and the electric traction systems that feed the transport on rails in direct current, presenting the electric energy s distribution in an underground system that uses the third rail to energize the trains. In addition, this analysis appreciates methods to design the rectifier substation, an electric installation essential in the conversion of the alternating current of the public network to a direct current used by the trains of the railway and the subway systems. With this approach, we perform the modeling and the design of the proposed subway system, demonstrating the process of calculating the capacities of the grinding substations to be used in a preliminary design phase.