This paper describes DC compensation systems using as a reference the most important cases reported in literature, and a prototype designed and built for the Metro system of Medellin.Three functions and their implications on the planning and operation of transportation systems that operate with grid-connected vehicles are discussed.First, the energy saving feature that allows a reduction in energy consumption in the system, because less energy has to be dissipated in braking resistors.Second, the voltage compensation capability that allows that voltage level limits are fulfilled through the overhead line, to extend the operative range distance.Finally, the application for optimization of the electric feeding infrastructure is explained.In the case of the prototype installed in the Metro de Medellin, the methodology for siting, sizing, design and implementation is presented.