The paper describes an integrated monitoring system installed in a reinforced concrete transit tunnel in Milan (Italy) where corrosion due to DC stray current is expected. The tunnel, about 9 km long, is divided into insulated sections, approximately 300 m long, obtained by reinforcement interruption and insulating membranes. Each section is monitored by means of reference electrodes to measure the potential and current probes to measure the current flowing within the concrete. All readings are collected in a centralized acquisition data system for interpretation and storing. The aim of the monitoring system is to control corrosion and to follow any modification of the reinforcement condition in order to anticipate corrosion attacks. The results obtained during commissioning and first period of working are presented and discussed.