Structural Health Monitoring (SHM) of bridges is getting special attention of infrastructure management agencies, due to its success in minimizing maintenance costs. The acquisition of structural behaviour data and its analysis to assess the structural condition are critical tasks of SHM. In this paper both are addressed. A cost - effective measurement equipment based on triaxial accelerometers is developed. It is coupled with an operational modal analysis tool that allows extracting natural frequencies, mode shapes and damping under operational loads. Both, hardware and software, are tested in a case study of a railway bridge of the Madrid Metro network. Other measurement equipments are deployed to compare results. Natural frequencies are calculated, showing a good correlation between the different systems. The obtained results provide relevant information about the global behaviour of the structure. Therefore, the proposed cost - effective solution could be used in a SHM system for continuous bridge monitoring.