----Autonomous decentralized system (ADS) architecture has been used mainly to realize high assurance control system. However, ADS architecture proposed previously is too simple to apply large-scale complicated system. In this paper, we clarify that large-scale ADS is represented by hierarchical three layers structure and many ADS sub-systems are connected by gateways and agents using railway transport operation control system (ATOS) as an example. ATOS is a system which has been growing and changing continuously. We show that both gateway and agent are effective for system assurance in functional system expansion for large-scale ADS.