© 2018 The International Masonry Society (IMS). The masonry viaduct at Marsh Lane is an important part of the railway network near Leeds, UK, dating from the 1860s. However, deterioration has resulted in notable deflections under train loads, which have concerned asset managers. Coupled with uncertainty regarding the true structural behaviour under serviceability conditions, this has led to detailed monitoring of the viaduct. This paper summarises the main conclusions of the monitoring installation before focusing on the evaluation of computational modelling of the viaduct, through comparison of modelling and monitoring results. In the monitoring scheme, fibre-optic cables containing Fibre-Bragg Gratings allowed measurement of dynamic in-plane barrel strains while digital image correlation captured displacements using commercial video cameras. The results illuminated a complicated three-dimensional dynamic response under train loading and highlighted the importance of interaction between adjacent spans. Separately, rail loading of the viaduct was simulated with a series of finite element models, each with increasing levels of complexity, to establish the relative stiffness contributions of various structural components. These models were then compared to detailed measurements from the real viaduct so that their validity could be evaluated. This approach revealed the impact of some common modelling assumptions and permitted assessment of nonlinear contributions to structural behaviour.