This paper presents an overvoltage analysis in the dc catenary of The Metro de Medellin railway system. The main objective is to study the sources of damages of motors in the trains. It was achieved a detailed study beginning with the direct measurement of variables and then the construction of a digital model in PSCAD-EMTDC. The simulations indicated a high incidence of braking overvoltages imposed on catenaries and insulations during regeneration. Literature and international standards on traction systems, concerning experiences and standardization of voltages, are analyzed.