Electrified transport is strongly assorted, due to long term development and use of diversified solutions, particularly in the scope of electrical engineering considering the following aspects: nominal voltage of a power supply system, vehicles’ main circuits and electrical drives. The problem of compatible work of varied railway systems in mutual interaction regions, despite of various studies, still appears a significant issue. The main purpose of the paper is to present the negative impact of mutual interaction of AC DC railway regions on the electrified railway lines and power grid. A Matlab – Simulink model of the DC and AC traction systems has been developed. In the article, apart from the main problems which could appear in the AC-DC transition zones, the voltage unbalance analysis caused by the AC traction substation is covered and the influence of asymmetric input voltage of the rectifier traction substation on the DC voltage quality is analysed.