Track circuit is a crucial part of the train control system. It is of great significance to establish a refined track circuit model for fault diagnosis, which is important for ensuring the train’s safety. However, the skin effect and frequency-dependent characteristics of rail component, etc., are not considered in all pre-existing track circuit models. It is one of the major problems in the fields of track circuit modeling, causing an obvious error, especially in the case of mixed frequency and wide frequency signals. In order to overcome such a defect, this paper first verifies the approximate linearity of the rail without traction current based on the finite-element simulation and practical measurement. Then via system identification, the transfer function of the rail as well as its equivalent frequency-dependent circuit model is established. Finally, the accuracy of the model is testified by the measured data. The measurement and simulation experiment results reveal that the presented model is of high enough accuracy in mixed frequency and wide frequency signal condition, which satisfies the requirement of track circuit fault diagnosis.