Over half the existing steel railway bridges in Japan have been in service for more than seventy years. Some of these bridges have severe problems including corrosion of the upper flange, fatigue and noise emissions. In this study, a method is proposed for the structural improvement of existing railway steel bridges through installing concrete slabs on existing steel girders. This method improves the load-bearing capacity of the bridges, extends their serviceable life, and reduces noise emissions. Feasibility studies for ease of application of the composite girders were made and the proposed method was verified to ensure it could be completed within the tight allocated time schedules. Furthermore we found through loading tests that the proposed method to produce composite girders also strengthened and increased the stiffness of the girders.