The Southeastern Pennsylvania Transportation Authority (SEPTA), the 6th largest transit system in the U.S. by passenger miles travelled, is among the nation’s commuter rail agencies in the process of implementing Positive Train Control (PTC) that must be completed by the Federal mandate deadline of December, 2015. SEPTA will be implementing ACSES (Advanced Civil Speed Enforcement System), originally introduced on Amtrak on portions of the Northeast Corridor (NEC), as an overlay system applied upon a base of Automatic Train Control (ATC) cab signaling. Every SEPTA train travels on Amtrak territory as a “tenant” at some time during its daily run, and consequently ACSES is required for interoperable PTC operation on Amtrak. The application of ACSES on SEPTA territory will be complemented by a simultaneous upgrading of existing and new ATC on SEPTA. Among the many decisions faced by the SEPTA PTC design team, the PTC communications systems design has proven to be challenging. The communications systems include digital radios, communications managers, ground based networks, office systems, network management and network security. Some elements have been designed from the ground up, with few commercial “off the shelf” components available to meet the design requirements. Moreover, SEPTA had limited 220 MHz spectrum available for a PTC radio network in a relatively compact geographic area of dense commuter rail traffic. The radio network and communications managers, in particular, are breaking new ground for U.S. commuter rail operations with adoption of Time Division Multiple Access (TDMA) digital packet radios operating in the 220 MHz spectrum, under the control of communications managers. This paper will present an overview of SEPTA’s PTC communications systems, and focus upon the design process, subsequent field testing, and the rationale behind key decisions reached by the design team, that led to the selection of the technologies and network topologies that will be utilized in meeting the implementation of PTC at SEPTA.