# The Use of Earth Observation Data for Railway Infrastructure Monitoring--A Review
## 3 EO-Based Railway Infrastructure Monitoring: Literature Review
### Comparative Analysis of EO Data Use in Railway Infrastructure Monitoring
By analyzing Envisat SAR images in the period of two years, ground deformations that resembled permafrost dynamics were observed. Although there was insufficient ground truth data to unambiguously link the signals to permafrost, the findings highlight the value of continuous satellite monitoring for ensuring the operational safety of the railway. Due to permafrost-related instabilities, the same track was also interesting to [PERSON] et al. [51], who applied small baseline subset interferometric synthetic aperture radar (SBAS-InSAR) technology to monitor, analyze, and predict surface deformation along the Qinghai-Tibet railway. A total of 1166 Sentinel-1A images from January 2017 to April 2023 were collected to obtain:1956 km of surface deformation along the railway. GNSS reference data confirmed the reliability and accuracy of the applied method. It was observed that deformation was mainly concentrated in the area close to the railway, with a maximum deformation rate of 26 mm annually. Furthermore, the Tent Mapping Sparrow Search Algorithm Long Short-Term Memory (Tent-SSA-LSTM) model was developed to predict future deformation for early detection and warning. To assess GD, [PERSON] et al. [53] discussed an integrated approach, i.e., fusion of synthetic aperture radar interferometry (InSAR) and ground-penetrating radar (GPR) technologies, for railway infrastructure monitoring. The proposed approach  
Figure 1: Line of sight (LOS) deformation velocity map: (**a**) before and (**b**) after 3-D geolocation improvement and (**c**) of persistent scatterers (PS) on rails. Subfigures (**d–f**) separately illustrate the zoomed-in map before and after 3-D geolocation improvement and of PS on rails over an example area (on the basis of [27]). was tested on a 10-km-long railway nearby San Severo (Italy) using SAR images from both the Sentinel 1A and COSMO-SkyMed missions. The fusion of InSAR and GPR data proved to be an effective tool, enhancing both the reliability and productivity of railway infrastructure monitoring. A similar approach presented by [PERSON] et al. (2016) explored the viability of integrating GPR for railway infrastructure monitoring. An experimental study carried out in Puglia (Italy) used SAR images from both the Sentinel 1A and COSMO-SkyMed. The experimental results validated the effectiveness of the combined data-fusion approach for assessing the health of transport infrastructure at a network scale. To complement on-site measurements of TZ, [PERSON] et al. (2018) proposed a system that uses InSAR technology for monitoring the transition zone containing the tracks on the steel bridge (embedded rail) and embankment (ballast track) located in Moerdijk in the Netherlands. The results of applying developed technology in analyzing TerraSAR-X satellite images were validated against two on-site optical measurement systems. Differential  
Figure 2: (**a**) AHN3 point cloud along the railway and its buffer zone with classifications. Estimated parameters on both CCS and TCS: (**b**) velocity map; (**c**) height map; (**d**) thermal dilation map. (**e**) Location of the railway (green line) ([PERSON], 2018). settlement induced by track components, such as expansion joints, outside of the bridge was also detected based on InSAR data. [PERSON] et al. [50] proposed a method for evaluating stress distribution in high-altitude Tibetan Plateau railway tunnels using subset synthetic aperture radar interferometry (SBAS-InSAR). It analyzes deformation data from Sentinel-1 images, obtained over the course of a year, of the Dongelu Tunnel on the China-Tibet Railway and compares these with high-frequency and high-precision automated vertical displacement measurements. A model to assess the vertical stress state under loading near the tunnel's entrance was developed, revealing slow and minimal surface deformation above the tunnel (1-3 mm/year).