An object is to provide an in-train transmission control system which can minimize delay in transmission of command data to devices further than the conventional art, in which a central unit provided in any car of a train formation composed of one or more cars and controlling operation of the entire train formation transmits command data to devices provided in some or all of the cars and in which the devices transmit device data to the central unit. The control system comprises relay device provided in the car, and line concentrator allowing the command data to pass therethrough, and when receiving device data from the devices, transmitting the device data to the relay device. The relay device aggregates the device data received from the line concentrator to transmit the aggregated data to the central unit.