With urbanization, the number of people living in metropolises is increasing, parallel to population growth, the demand for rail transportation systems, which has a very important place in urban transportation, is increasing day by day in populous cities. The traffic density, which will increase in parallel with the increasing demand on rail, makes the signal systems very important in terms of their impact on rail safety, capacity and efficiency. Today, communication-based train control (CBTC) is the preferred signaling system in many rail transportation systems. CBTC consists of some subsystems such as Automatic Train Protection (ATP), Automatic Train Control (ATS) and Automatic Train Operation (ATO). The subsystems have some specific, defined tasks. In performing these tasks, it is expected that the desired safety functions will be implemented with the highest level of confidence without being compromised by the increasing requirements from the subsystems. Nevertheless, the overall design of these systems may require to be modified to meet the new necessities. In this case, it is of great importance to control and model the subsystems while considering different standards. In this study, the subsystem of CBTC, ATP, is modeled with finite state automata with the discrete event system approach, and the supervisory watchers of the subsystems are designed, shown to satisfy the controllability and nonlocking conditions, and the obtained results are matched with the model of the monolithic approach.