In the EU railway network, a multitude of railway companies runs their rolling stock on tracks which are not in their own responsibility. Consequently, permanent monitoring of security-relevant parameters becomes an issue of increasing importance. In order to characterize the status of the catenary – current collector interface, compliance with the specified contact force between them is a generally agreed-upon quality parameter. We report here on an approach to monitor contact forces (without correction of dynamic parts in this first step) inline using a potentially low-cost fiber optic sensing system which is insensitive to the high electrical potential and to field changes. It was implemented and tested successfully on regular trains by replacing the current collector with a sensor-embedded one, and by adding a small-sized signal processing unit in the driver cabin. In this article, we describe the construction and application results of the system and discuss its advantages and limitations. Sensor characteristics are investigated with the help of a mechanical model. The system has shown its potential during test drives on railway tracks in Switzerland.