Marsh Lane viaduct is a typical example of a 19th century brick masonry railway arch in the UK. It frequently carries passenger trains to and from Leeds Station. This paper broadly discusses the sensing techniques and associated analysis procedures used to (i) identify the reasons for existing damage, (ii) quantify their impact on the dynamic response of the structure and (iii) measure degradation of the response over a period of one year. To identify existing damage, distortions in geometry of the structure are examined with new point cloud processing techniques. With the aid of limit analyses, these distortions are interpreted, and past support movements which may have caused the distortions are identified. Then, to measure the dynamic response of the bridge, quasi-distributed fibre optic strain sensing and digital image correlation displacement measurement techniques are used. These highlight the increased dynamic response around locations of existing damage, and point out to the global mechanisms of response that could propagate damage. Continuous fibre optic strain measurements between November 2017 and 2018 are then discussed to investigate the ongoing deterioration.