The railway signaling system has recently achieved realization of new functions by using assurance technology. By constructing systems in stages, making both control data and information data coexist on the same network, and realizing coexistence of both old and new systems, we succeeded in introducing a large-scale system as well as system change. This method also improved services to customers such as a decrease in train delay and continuation of train operation. Assurance technology can now be applied not only to systems in which we have already used this technology in systemization, but also to many other railway systems. On the other hand, when we try to change a complex and large-scale system, it is difficult to do so in order to accommodate new needs with completely new functions, since the new system will be affected by the existing system. We reviewed ways to solve this problem by using assurance technology