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The ATCS Communic,ations System, which provides for the information flow in the North American Advanced Train Control S stem, is resented in the context of the overall A k S plan. ?he features and functions of .the nodes,in the communication system, and their connectivity, are defined. The system is based on the OS1 model; and the protocols used are described with particular emphasis on the radjo data link. .Special eatures ,of the system, including a description of how vehicles are tracked, are described.