Curve squeal is due to the imperfect curving behavior of rolling stock and is typically related to the inner leading wheel, which slips laterally on the rail. This may cause a non-linear instability leading to self-sustained wheel vibrations and by consequence, loud and tonal sound emission. This is perceived as highly annoying by line side residents and is a major noise problem for railways in urban areas. In order to evaluate potentially critical situations (e.g. change of rolling stock) or to design mitigation solutions (e.g. wheel dampers), VibraTec developed an engineering tool called SONIA (Squeal Occurrence NoIse Analyzer). Based on the characteristics of wheel, track, as well as the kinematic, contact and friction conditions, SONIA can predict if curve squeal is likely to occur. A stability analysis of the mechanical interaction of the wheel-rail system is carried out, using linearized contact forces. If curve squeal occurs, the sound power associated to each unstable frequency is estimated: the amplitude of the limit cycle is calculated through a balance between the injected and the dissipated power within the wheel-rail system.