In this paper an experimental work regarding power quality in existing dc traction systems is presented and evaluated. The measurements were performed in two substations of the Athens-Piraeus Electric Railways (ISAP), according to the European standard EN 50160 and EN 61000, using 3 wire connection systems. The examined traction system use 600V DC. The main supply systems usually consist of a medium voltage substation feeding a rectifier which provides the dc power supply of the traction motors. The DC power supply is provided using grounded negative bus. The three-phase rectifier (controlled or uncontrolled) produces harmonics induced to the AC network. The measurements were performed in two substations one old with oil insulated transformers and old 12-pulse rectifier and in one recently refabricated with dry type transformers and new rectifier.