The footbridge over the railway at Santos Station in Lisbon is a steel structure supported by circular columns at its ends, with access stairs extending perpendicularly to the main axis of the bridge. In June 2022, an excessive concentration of pedestrians caused deformation in the cantilever supporting the landing of the north staircase, leading to the installation of BB20 bars to reinforce the landing support. Since then, foot traffic has induced excessive vibrations in the footbridge. To investigate this phenomenon, ambient vibration tests were conducted to identify the cause and assess compliance with the comfort criteria outlined in the National Annex of NP EN 1991–2. The dynamic characteristics of the footbridge were experimentally determined, and the results were used to calibrate a finite element numerical model. Analysis based on this model confirmed that the frequency of the first longitudinal vibration mode falls within the resonance risk range for the second harmonic, with the most critical condition occurring when the North landing is supported by the additional bars.