Steel cables and tendons are commonly used in reinforcing steel beams as well as in concrete beams. However, the structural detail of cable anchors in steel beams tends to be complicated, and the effect on reducing live load stresses is not significant because of the relatively small stiffness of cables and tendons. On the contrary, by using high strength steel plates instead of cables and tendons, structural detail of the anchor area becomes simpler, and live load stresses as well as dead load stresses can be reduced in steel beams because of the relatively large stiffness of steel plates. In this study, the steel plate prestressing method is applied to beam specimens and intermediate floor beams of a steel railway through girder bridge. The behavior of the reinforcing steel plates and reinforced steel beams is monitored during prestressing and live loading, in order to assess the effects of prestressing and reinforcement. The study confirmed that these effects are beneficial to the performance of steel railway floor beams.