In this study, we present an analysis of the causes of a landslide along a railway track in the Polish Lowlands. The landslide damaged the railway track and caused significant material losses. Digital models of the terrain surface before and after the landslide were elaborated. Remote sensing using LIDAR aerial technique and a terrestrial laser scanner was performed to determine the morphology. Soil mass behaviour was analysed by 3D numerical simulation. A numerical model was created based on geotechnical tests. Taking into account the behaviour of the dry and wet models, the numerical simulation showed the most probable scenario of mass movement. The main reasons for the landslide were rainwater infiltration in the track basement and the unfavourable morphology of the area on which the railway embankment was located. The study demonstrates that combined methods—laser scanning, geotechnical testing of the soil material, and 3-dimensional numerical simulation—enabled the assessment of the causes of the analysed landslide.