Recent advances in optical sensing and signal processing technologies have resulted in strain sensing systems that can accurately capture the very small dynamic strain levels occurring in civil structures during ambient excitation. In particular, it has been demonstrated in a laboratory environment that the combination of fiber Bragg grating (FBG) sensors with an acquisition system that interrogates those sensors with a swept monochromatic signal, can accurately capture RMS strain values with an order of magnitude of 0.01 microstrain. In the present work, the potential of this novel sensing technology for civil structural health monitoring is explored for a steel bow-string railway bridge that has been monitored for over one year. Both sides of the bridge deck have been instrumented with chains of multiplexed FBG sensors that are fixed to the bridge at discrete points. In this way, dynamic macro-strains are measured in a continuous way along the entire length of the bridge. During the monitoring period, the bridge underwent a retrofitting operation, in which all connections between the hangers and the bridge's deck and arch were altered. The influence of this retrofit on the monitored eigenfrequencies and strain mode shapes has been investigated and compared with their regular environmental variation. As the retrofitting resulted in global mass and stiffness changes of the structure, it is found to have an influence mainly on the eigenfrequencies, except when it induces an interaction between previously well-separated modes: in that case the strain mode shapes are also affected.