## 5 Conclusions  
The vehicle-based iSHM method was applied to determine the natural frequencies of railway bridges by measurements of the train vibration responses during the bridge crossing event only. Numerical train crossing simulations were carried out for an existing steel deck bridge as well as two further fictitious railway bridges, considering a multi-body model for the traveling train and a simply-supported single-span Euler-Bernoulli beam. In order to validate the results of the numerical simulations, in situ tests were executed at the existing steel deck bridge, and both the steel bridge and the traveling train were instrumented with accelerometers. At first, forced vibration tests were performed to determine the natural frequencies and damping ratio of the steel bridge. Second, the vibration responses of the train and the bridge were measured during the train crossing events. It could be shown that in the numerical simulations as well as in the in situ measurements, the FFT-transformed vibration responses of the bogie frame \(b2\) during train crossing allow the most reliable extraction of the first bending frequency of the considered railway bridge. In summary, the following outcomes of the research are drawn:  
1. The executed analyses--shown here only for a particular bogie frame of the considered train, for a particular traveling direction, and three exemplary realistic railway bridge structures--indicate a strong similarity in the dynamic behavior of the bridge and the bogie frames during train crossing. This is particularly evident in the frequency domain, although identifying the bridge's natural frequency is only possible when analyzing a wider range of speeds, including at least one resonant speed;
2. At crossing speeds of the trains, which are not very close to resonance speeds or where a cancellation speed is close, the frequency components resulting from the excitation frequencies dominate the acceleration response of the bridge structure and bogie frame. The same applies to cases where the bogie frames, for which the natural frequency of the primary stage is--as in the case of the existing steel deck bridge--also in the vicinity of the first bending frequency;
3. The conducted forced vibration tests of the existing steel deck bridge and, in particular, the bridge's natural frequencies and damping ratio determined turned out to be an important basis for the reliable identification of the bridge's first bending mode by application of vehicle-based iSHM method;
4. In situ measurements of both the train and bridge vibration responses were performed during the train crossing events, and thus, it could be shown that the first bending frequency of the bridge \(n_{d,1}=4.52\) Hz (determined by forced vibration testing), was dominantly excited through the crossing train at 135 km/h;
5. The first bending frequency of the bridge was clearly determined by the application of the vehicle-based iSHM method, considering the frequency response function of bogie frame vibrations during the train crossing event at 135 km/h. The train-identified bridge's bending frequency was identified with \(n_{d,1,T1}=4.38\) Hz, which is in accordance with the numerical results and around 3% smaller than the value determined by forced vibration testing. Finally, it is concluded that the application of the vehicle-based iSHM method provides a promising tool for identifying the dynamic characteristics of railway bridges by only measuring the vibration responses of the crossing train. In particular, potentially identifying changes in the natural frequencies of the structure, which can indicate possible damage to the structure during regular railway operations, is a promising application possibility for predictive maintenance to be explored with further development of the method. However, challenges were detected regarding the reliable identification of the bridge's natural frequencies from the bogie frame vibration responses only, without having the results of the bridge's frequencies from performed in situ measurements.