Because of the heavy, periodic loading caused by trains, the dynamic response of a Rail Bridge is often the most useful means of assessing its structural health. Whilst this is done anecdotally by drivers regularly as they report changes in ride comfort, there are times when a more localised and specific, or more accurate assessment is required. Track Geometry Vehicles such as the New Measurement Train in the UK are run continually to assess condition, but these are expensive, often run on a set timetable, and only pick up the response of the track overall, not distinguishing between voided sleepers and an unusually bouncy structure beneath for example.
In the UK, rail bridges are usually of either masonry or steel/wrought iron construction, and the majority are over 100 years old. Steel bridges in particular show substantial movement under dynamic load, but both types are able to be accurately monitored by the Imetrum system, using the brick pattern or rivets as virtual targets. Real-time measurements are typically taken at a rate to match line speed (e.g. 120 Hz is suitable for HSTs travelling at 120 mph), allowing the displacement of the structure to be accurately measured. All this can be done at any point that can be seen in the video image, with remote set-up being possible (meaning structures spanning busy roads or waterways can also be easily monitored, and there is no need to access track).
In the case illustrated below a steel bridge spanning a local road was monitored at a 100 Hz frame rate as Loaded Yeoman Hoppers passed over carrying quarry stone. The pattern of the rivets was used by the software to detect movements in the overall structure. The trace shows different displacements caused by axle loadings on the lead waggons compared to those at the rear of the train.