The radio-based Data Communication Subsystem (DCS) between trains and wayside is a key factor for safe and efficient operation of a Communication-Based Train control (CBTC) system. To provide means for a reliable and optimized design of the DCS RF segment, this article presents an in-depth discussion of the UHF/SHF radio propagation process in tunnels. To present a flexible and realistic radio propagation prediction model inside a tunnel environment the framework of modal analysis has been combined with the ray tracing technique and a heuristic approach has been adopted to assimilate horizontal curvatures in the tunnel geometry. The theoretical radio propagation loss predictions from the proposed model are compared to field measurements collected in Toronto subway tunnels and satisfactory agreement is observed.

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