This contribution is rather different from the other articles we're run in this department to date. Here, software volume isn't an issue as there is only one Tokyo-instead, we look at the incremental development of a network of systems controlling railway lines, which requires adding value without disruption. This particular scenario contains many interesting facets, including the use of COTS and refactoring. We're indebted to Takao Futagami for facilitating this contribution.