This paper describes a diagnostic system for overhead line monitoring; it is based on the measurement of collector accelerations, and aimed at improving current scheduled methods for catenary and pantograph maintenance, making condition-based maintenance possible. The proposed setup is inexpensive and easy to install on in-service commercial trains. Meaningful indicators of pantograph/catenary interactions are obtained by the processing and analysis of data on collector accelerations; summarized values are computed in real-time during the train run and compared with alarm limits