This paper presents the modeling and simulation of voltage unbalance for AC electric railway systems. The development is based on MATLAB/Simulink program to determine voltages at the substation. The voltage unbalance factor (VUF) is used for this evaluation. Suvarnabhumi Airport Rail Link (ARL) in Bangkok, Thailand, is chosen for test. This test system is supplied by a cyclic winding connection of two single-phase traction transformers in which its primary side connected to the 69-kV transmission system of MEA (Metropolitan Electric Authority) power grid. The AC railway power feeding is a direct feeding scheme via 25-kV overhead catenary system. The total service distance of the ARL is 28.298 km with 8 passenger stations. The multi-train movement simulation is developed and embedded into the MATLAB program. The simulation results show that the operating voltage and the VUF during the train service can be obtained.