In a deregulated EU railway market, monitoring the vehicle and infrastructure interfaces is mandatory for the enhanced availability of operation and for reducing costs. Therefore, infrastructure managers need monitoring tools on overhead contact lines (OCLs). We know from earlier investigations that a measurement of contact forces alone is not sufficient [1]. In this study, we introduce a system which is fast enough to detect short disturbances and which can be used with regular trains. It is based on fibre optic sensors integrated with conventional current collectors (CC). The system is designed to monitor hard and soft hits on the current collector in horizontal (driving) and vertical (contact) direction. It was systematically tested in the laboratory as well as in test runs on commercial railways in several countries. With its help, a variety of minor as well as serious defects have been discovered at the CC–OCL interface.