The presence of large paleo-landslides can hinder the construction of railway lines if they cause an alteration of the natural balance of the slope, significantly increasing the cost of the project. During the construction works in a section of the Madrid-Asturias high-speed railway line (Spain), a large-scale hillside instability affected 460 m of the section. The ground movement began ten months after the start of the excavations and remained active throughout the observation period (2008-2010). Data provided by fieldwork, boreholes and instrumental monitoring has allowed the investigation of the geological units involved, together with the geometry and the kinematics of the mass movement. This landslide involves a Paleozoic basement with an estimated volume of 4,400,000 m3. It shows low displacement rates (<45 mm/year) and was accommodated on a single surface of rupture, that reaches more than 60 m in depth. The kinematics is favoured by structural and lithological conditions, there being a strong rheological contrast in the lower part of the mobilized ground. This movement is linked to a large paleo-landslide that was partially reactivated when excavations undermined the lower slopes of the hillside. Technical solutions for the stabilization have already caused an additional cost of 17% over the initial budget.