Effective engineering design of structures requires a thorough understanding of the groundwater conditions of the substrate. In some situations, a three-dimensional survey is necessary. Landslides are examples of such cases. They are complex phenomena, and the main factors significantly influencing their behaviour over time are changes in slope geometry, inclination and water conditions. The article discusses the reconnaissance of the substrate structure in an area threatened by mass movements along a modernized section of a railway line. The analysed area is located in the marginal zone of the North Polish glacial moraine. The geological structure of the substrate consists of: glacial tills, glaciofluvial sands, lacustrine clays, and organic soils found in periodically waterlogged areas and depressions in the terrain. Colluvial deposits, mainly consisting of clayey formations, occur on the slope of the escarpment. Surface geomorphology was interpreted using LIDAR data and field observations. Two-dimensional and three-dimensional electrical resistivity tomography (ERT) was used to obtain a detailed subsurface image, which was verified by borehole drilling and laboratory analysis of soil samples for physical properties, including grain size distribution and plasticity, as well as mechanical properties of soils. This research enabled the creation of a three-dimensional substrate model, showing the spatial distribution of colluvium and areas at risk of active landslides. The results indicate that an integrated approach, combining geophysical imaging and geotechnical reconnaissance, allows for a detailed understanding of the structure and lithology of landslide areas.