Masonry arch bridges constitute a significant portion of the European rail network. Many of these structures have been in operation for over a century and have demonstrated resilience in undertaking increasing rail loads to meet modern traffic demands. However, today they suffer from localised failures due to the combined action of material weathering, fatigue loading, and pier settlements, which raises serviceability concerns for infrastructure managers. Furthermore, their structural performance assessment is particularly challenging due to the high heterogeneous and discontinuous nature of ageing masonry. Thus, to underscore the importance of long-term condition assessment of railway infrastructure, this paper presents a comparative analysis of monitoring data gathered from a network of fibre Bragg grating (FBG) sensors installed on a deteriorated Victorian railway viaduct over an 8-year period (2016 to 2024). The study presents a methodology for assessing mechanical damage by observing variations in the dynamic strain response during train loading, beyond normal seasonal effects. Signal processing and statistical analysis of the most recent dataset (2024) confirm that the overall dynamic deformation of the bridge remains stable, although new localised strain variations are observed along the transverse sensor arrays near the most damaged pier of the bridge. This underscores the importance of having a fine network of sensors to capture local response variations and identify critical regions undergoing potential deterioration. To this end, the fibre optic monitoring system installed has been highly consistent over the years, making it an attractive option to support the long-term monitoring of ageing infrastructure.