This paper describes a system modeling method and Its application to a new train control system based on the train detection COMBAT that uses radio communications. In the System modeling process, we paid attention to the function of train detection. We propose a basic model that describes the state of detecting point with three parameters. By using this model, we can define and classify the results of train detection at the detecting point before performing safety analysis. Based on this definition, we can analyze various states of the system resulting from actual train movement and state of equipment. The method we propose in this paper enables us to perform system behavior analysis systematically and efficiently and clarify the point where we should pay attention in the actual system operation.