Large amplitude vibration is observed at some railway bridges. Train axle loads cause resonances. To address this problem, the use of active mass damper is numerically studied. Unique characteristics of this problem are as follows. The stroke of the dampers is limited. The loading determined by the wheelbase combinations is non-stationary. Both double-passage and single-passage cases need to be considered. Pseudo-static displacement of the bridge exists and its effect at train entrance and exit needs to be considered. The frequency of the bridge changes over time due to environmental changes. A variable-gain feedback control is employed with design parameter identification through genetic algorithm. The active control is numerically shown to reduce the train-induced vibration.