In this work a stiffness transition zone between surrounding soil and a bridgestructure is evaluated, showing how it affects the bridge dynamic behavior. It ismade to give a better understanding of the transition zones impact on a bridge. Abridge structure is complex, depending on several interacting parts as soil-structure and rail-ballast. Simplifications are made in the work with the ground rule to simplify calculations but minimizing impact on basic structural behavior. In principal the impact of a transitions zone has been studied by creating a representative model, verifying the model, performing a parametric study and then evaluation using design of experiment. Limitations are made by using a 2-dimensional bridge in a single span, modeled linear elastic. The load is considered as a series of moving point loads. The result shows that a transition zone impact the structures dynamic behavior. Itis also shown that there exists a point where increasing the length of a transitionzone gives no further positive contribution to the structure.Within the scope of this thesis there has also been an investigation made on the topic of soil-structure interaction. This mainly on the fundamental property of eigen frequency. It is shown that surrounding soil of the modeled bridge decreases the first eigen frequency with respect to vertical deflections.