Culverts are another form of Bridge (Underline). They allow streams and other small watercourses to pass beneath (under) the railway tracks.
For the purposes of the SCRCA Project, structures have been classified as 'culverts' if that is how they are marked on the 1911 landplan. However, the criteria used by the engineers / draughtsmen when deciding whether to apply the term culvert rather than bridge is not apparent. A quick review of the default images will show that the choice seems to have had little, if anything, to do with the structure's size, design or construction materials. Most culverts include an engineered 'floor' of some kind to direct and speed-up water flow and to eliminate the risk of undercutting. (The latter can be a problem when bridge spans are supported on piers that have been set into or close to the bed of the watercourse.)