A transmission system includes: a hub to receive a control command from two or more controllers that control operation of an in-vehicle machinery installed in a train, and to control an output of the control command on the basis of a priority assigned to the control command, and a system-controller to determine an operating state of a controller that is a source of the control command, the system-controller being capable of changing a priority assigned to a control command transmitted from a controller whose operating state has been determined, on the basis of a determination result.