The onboard bridge deflection estimation method of railway bridge based on track irregularities have been developed. However, the existing method, for a single bridge, could not apply to 2 bogie track inspection vehicles since the track irregularity measured by it is physically affected by the deflection of the adjacent bridge. In this study, a novel bridge deflection -track irregularity conversion program is developed to cond uct fundamental investigations to establish the bridge deflection estimation method based on track irregularities measured by 2 bogie track inspection vehicles. The program outputs the difference between track irregularities (DTI), which cancels out static track irregularity and depends only on bridge deflection. The effect of the adjacent bridge on the DTI of the target bridge is investigated through the numerical calculation in the section which consists of 7 consecutive bridges of 13m beams.