This very famous and fascinating structure dates back to the eleventh century and has a total length of about 100 m. It is made up of four semicircular arches, the greatest of which has a clear span of 38 m. The bridge, whose piers are immersed in an artificial basin created by the water of the Serchio River, hosts in its right abutment the railway connecting Lucca with the Appennine passes. During the MONSTER project, the bridge has been instrumented with four high-sensitivity triaxial seismometric stations, made available by the Seismic Observatory of Arezzo (INGV), left active on the bridge for about 8 hours, on the 16th of June, 2015. The sensors were moved in different positions on the bridge’s deck, in order to improve the description of the mode shapes. Data coming from the monitoring system have been analysed via the Stochastic Subspace Identification and the Enhanced Frequency Domain Decomposition methods. Finally, a finite-element model of the bridge has been created via the NOSA-ITACA code and updated to fit the experimental data. Dynamic analyses of the bridge’s response under the self weight and during the transit of a cargo train have been performed, by applying to the model an acceleration time-history recorded on the railway in June 2006.