The condition monitoring of concrete in a railway tunnel presents a series of challenges in sensor design especially where the sensors are to be installed post construction. There is a need to minimise the intrusion into the tunnel lining whilst maintaining an ability to detect corrosion conditions on both interior and exterior layers of reinforcement. The severe time and physical constraints on the installation process and the overriding need to maintain electrical safety and electromagnetic compatibility with the railway environment further constrain the design. This paper describes the development and validation of sensors to meet these constraints and the integration of these into networked systems. The fundamental principles of the system and practical aspects of the work are detailed. Expected results from the system are discussed for future comparison with field results and a parallel laboratory study program.