This study investigates an existing steel plate girder railroad bridge in order to monitor static and dynamic responses using fiber Bragg grating (FBG) sensors. This paper also presents an experimental technique to estimate the vertical deflection of the bridge using FBG sensors. Seven FBG sensors are multiplexed in a single optical fiber and installed in parallel pairs along the length of the bridge, with one set on the top flange and the other on the bottom flange. The train passes over the bridge at different speeds, ranging from 10 km/h to 90 km/h, so as to monitor the dynamic response of the bridge. The results show that the proposed instrumentation technique is capable of estimating the deflection of the bridge for various loading conditions, which is crucial in structural monitoring.