Stresses in bridges due to traffic loading need to be determined as accurately as possible for reliable fatigue safety verification. In particular, the dynamic traffic effect due to running vehicles has to be considered in a realistic way. In-situ measurements of the dynamic behavior of a one-track railway bridge have been performed to analyze the complex elastic dynamic system consisting of running trains—railway track—bridge structure. The main causes for dynamic effects for fatigue relevant bridge elements have been identified to be the vertical track position, i.e. track irregularities. The dynamic behavior has been modeled with sufficient accuracy using simple models when fatigue relevant dynamic effects are studied. The results of this study allow for the consideration of realistic dynamic amplification factors for fatigue verification.