A new microwave communication system for centralized train traffic control, utilizing TE01 mode propagating circular waveguide as a transmission line is discussed. A new microwave communication system for The system was developed for control of 100 ultra-high-speed trains on a railroad 500 km in length. The aluminum waveguide of 132-mm ID, with measured attenuation of 2.0 db/km on 7500-Mc band, with a helical waveguide as a mode filter at 100 m intervals is laid alongside the railroad. Signal transmission to the running trains is achieved through directional couplers in the circular waveguide and auxiliary waveguides for radiation, without signal dropout. Radio equipment for the trains and radio repeater features special simultaneous amplification of incoming and outgoing signals by the same TWTs, and varactor diode multipliers. The system consists of a total of 600 telephone channels to provide 6 telephone channels to each train, to be used for high-speed data transmission and telephones. A new microwave communication system for Experiments made with a waveguide run of 1 km in length laid alongside the Hachiko Line of the Japanese National Railways have proved its capability for centralized train traffic control. Codes transmitted at speed of 2000 bauds during the test train running had no error, and a total of 573,000 characters was transmitted. Good TV transmission was also attained.