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The importance of the specification of train control system is increasingly recognized and gained more attention in signalling field in China as the specification is the basis to ensure that the signalling system supplied by manufacturer meet the requirements of the railway administration, for example, the requirement for the interoperability of the system. The specifications which are described in natural language are probably deficient and it is inadequate that the specifications are checked only based on the experience of experts. In this paper, a modelling method was applied on the description and the verification of the System Requirement Specification, SRS, of train control system. The Specification and Description Language, SDL, was used to describe the functional behavior of the onboard equipment which is defined in the SRS. First, we defined the principles of modelling which are summarized for the purpose of the interoperability validation. Then, we applied a top-down hierarchical approach to model the functional behavior. The model started from the system level to describe the interface and refined in the block level to show the interaction of system scenarios and detailed the state transition and the working processes of the system in the process level. Debugged the SDL model, we validated the model in Telelogic Tau tool to find the problems of the SRS. The results showed that the ambiguous terms and the incompatible descriptions of the SRS can be found. It can be helpful for the modification of the SRS and the quality of train control system further.