The HSL-Zuid is a 100 km high-speed line, running from Amsterdam via Rotterdam to the Belgian border. From there it links through to Antwerp, Brussels and Paris. The Dutch part of the line connects to existing tracks at five locations. At these points, the interlocking systems of the existing network are linked to those of the HSL-Zuid system. The existing Dutch rail infrastructure is managed by ProRail. ProRail determines the conditions that must be met by systems that are to be connected to their existing systems, including interlocking systems. The HSL-Zuid organization has to ensure that these connections are demonstrably safe and reliable. This article describes the testing of the connection between the HSL-Zuid interlocking and that of the existing infrastructure. It explains the structure of the tests, their content, the division of responsibilities between the parties and how a demonstrably safe overall system is created.