## 6 Conclusions  
This paper describes some experiments aimed at monitoring the dynamics of a multiple-track railway bridge (NDHRB) using a microwave radar interferometer named IBIS-S. Three stations (S1, S2 and S3) with four radar views (view S1, view S2a, view S2b and view S3) were set to monitor different parts of the bridge dynamic and monitor the dynamic responses under different loading cases: ambient noise and wind, high-speed train passages and metro passages. The radar was positioned underneath the bridge, looking the bridge longitudinally and perpendicularly to the bridge. Lidar point clouds measured by TrimbleTM TX8 3D scanner were used for interpretation. The RAR data were compared with those coming from the bridge SHM system. The RAR data showed better results in describing the dynamic behavior of the bridge: the dynamic details and the maximum of bridge displacement can be properly captured using the RAR data. In addition, the spatial features of the bridge deck deformation under the eccentric train loads was properly described using the RAR system. These advantages indicate that microwave interferometry is a useful technique for monitoring the dynamics of bridges with multiple tracks.  
Here below are summarized the main conclusions related to the experimental results of this work:  
Figure 19: Vertical displacements in (**a**) view s2b case 2 track 3 and (**b**) view s3 case 4 track 3. the ’left’, ‘middle’ and ’right’ are the radar bins selected at the corresponding position in the bridge lateral.  
1. The radar measurements at S1 show that, for a 336-m steel truss span, the magnitude of ambient vibration in the bridge vertical is within \(\pm\)0.3 mm. This response agrees with the rigid characteristic of the railway bridge at hand;
2. The experimental results at S2 show that, for a 192-m steel truss span, the vertical displacements induced by high speed passing train (250 km/h) are less than 12 mm; the pattern of the displacement time history is related to the track of train and its duration depends on the period during which the train is passing on the main bridge. Good consistency in all the representative loading cases was achieved with the in-situ SHM system data. A more detailed displacement behavior is achieved from IBIS-S due to its higher sampling frequency and the maximum displacements are captured;
3. The lateral behavior of the bridge can be studied when setting the radar perpendicularly to the bridge, while the traditional way of sensor setting (parallel to the bridge) cannot. Measurements of the bridge deck perpendicularly to the bridge (S3) show that, for a multitrack steel truss bridge, the vertical displacements are not uniform in bridge lateral direction larger displacements are monitored in the track where the train is passing, while minor effects are present on other tracks.  
[PERSON] conceived and designed the experiments; [PERSON], [PERSON] help to analyze the data; [PERSON], [PERSON] and [PERSON] contributed the field campaign; [PERSON] prepared the manuscript, [PERSON], [PERSON], [PERSON] and [PERSON] revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the Fundamental Research Funds for the Central Universities (2018B18814, 2018B699X14) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0619). The CTTC activities were partially funded by the Spanish Ministry of Economy and Competitiveness through the DEMOS project \"Deformation monitoring using Sentinel-1 data\" (Ref: CGL2017-83704-P).  
We thank [PERSON], from Beijing MAG Tianhong Science & Technology Development Co., Ltd., for collecting the Lidar data and [PERSON] from Southeast University, [PERSON], from JSTI, for their support of the ground base radar campaign. The hydrostatic leveling data were provided by China Railway Major Bridge (Nanjing) Bridge and Tunnel Inspect & Retrofit Co., LTD.  
The authors declare no conflicts of interest.