It is well documented that many old structures were overdesigned decades ago and could operate under conditions that were not even thinkable at the time of construction. But while the structure might look in relatively good shape, change in the use of the bridge (implementation of the high-speed train for instance) might easily be the factor that turns an overdesigned bridge into a potentially failing one.
The railroad owner mandated Roctest to support them with their study of that bridge, performing a short period of tests to test it under the current usage. The plan was to monitor in real-time (dynamic monitoring) over a weekend each train passage and see how bridge elements were behaving in comparison with 
theoretical value, given after a Finite Element Analysis was performed on these same bridge elements.
The use of Fiber Optic sensors, model MuST (using FBG technology) was chosen for its easiness of installation/de-installation and its capacity of real-time (500Hz) simultaneous readings of multiple sensors.
After the end of the testing period, all sensors were quickly removed, and no signs of any monitoring were left on the structure. Results clearly show the difference between strain caused by the locomotive and the wagons behind. It also clearly the maximum strain each monitored bridge element where under, making the comparison with theoretical values very simple.
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