The conventional ground-based train control on-board computer system for keeping the fixed distance between the trains cannot satiify the high density trains operation Then Autonomous Decentralized train control on-board computer system is proposed, where each train has autonomy to detect its position and its speed and to judge its speed for stifety and punctuality. For realizing these real-time on-board computers and their network system, the train operation cannot be stopped during their step-by-step construction and their test while both of the conventional and new systems have to coexist. The autonomous on-line expansion and test technology is proposed; where each train has autonomy to select to receive the data based on its content and to judge its nomlai or test operation In the process of replacing the conventional system with the new, two assurance technologies, where the networks based on the content code commUllication are integrated to coexist their different data and they are separated but connected through the gate way, are proposed These technologies are evaluated by how much to keep the total system operation These technologies are shown to be effective at the experimental railway, and it is now applying at the Keihin Tohoku line in the Tokyo metropolitan area .