Part I of the Paper gives a brief description of an automatic train protection (ATP) system for a mass transit railway, and indicates some problems that may arise because of inductive interference caused by the presence of reinforcing steel in the structures supporting the track. Part II deals with two months of study capable of yielding quantitative estimates of the interference effects in a reliable and systematic manner. In the first method a full-scale simulated track system was built and, based on suitable instrumentation, the physical parameters as well as the relationships governing the inductive interference were determined. The second method relies on establishing a mathematical model that invokes basic electromagnetic field theory and transmission line theory to arrive at a generalised solution. The results obtained with both methods are presented and compared.