Abstract Abrupt track vertical stiffness variations along railway tracks can lead to increased dynamic loads, asymmetric deformations, damaged track components, and consequently, increased maintenance costs. The junction of slab track and ballasted track is one of the existing areas where vertical track stiffness can suddenly change, therefore requiring a transition zone that smoothes the track stiffness change. One of the methods for constructing the transition zone at the junction of slab and ballasted tracks is to install auxiliary rails along the transition zone. In the present study, the dynamic behavior of this type of transition zone was evaluated by a train-track interaction model. For this purpose, a 3D model of the railway track was made, representing the slab track, the transition zone, and the ballasted track. Then, the modeling results were validated by the results of field tests. Afterwards, in order to study the dynamic behavior of the transition zone with auxiliary rails, different sensitive analyses, such as vehicle speed, vehicle load, number of auxiliary rails and railpad stiffness, were performed with the model. The obtained results showed that the use of auxiliary rails reduced the rail deflection variations along the transition zone from 35% to 28% for low and medium speeds (120, 160, 200 km/h), and from 40% to 33% for high speeds (250, 300 km/h).