For railway radio communication systems between train and trackside, utilization of the 90 GHz band is considered in Japan. To design the system and evaluate the interference to other radio systems, propagation characteristics should be clarified for various usage environments. In this paper, propagation characteristics of path loss and r.m.s. delay spread are described based on measurement results in a viaduct environment. Path loss coefficients are extracted at different antenna heights compared with the side wall height of viaduct for a link budget design and it is validated by ray trace simulation. The cumulative distribution of delay spread is also analyzed for physical layer design. The system parameter of railway communication system in 90 GHz band is suggested by considering these propagation characteristics.