The re-signalling of metro networks includes some specific risks. The main risk is that of disrupting service during the critical migration phase from the old to the new system. New signalling system technology must be easy to overlay on the existing system. Interference of the new with the existing system must be minimized in order to avoid disturbing the system in operation and creating passenger service disruptions. On a practical level, it is essential that primary train detection and data communication from the new signalling system be independent of the existing track circuit detection system. This article describes a solution.