A device (40) for the detection of mechanical forces exerted on a rail such as a railway rail (17), comprising a mechanical detection element (41), which is coupled to said rail (17), and sensor means (45). The sensor means (45) are built up of at least two sensors that are in mechanical contact with each other, which elements are each provided with terminal electrodes, wherein one element is of a first type, arranged for exerting a mechanical force under the influence of an electric signal applied to its terminal electrodes, and wherein another element is of a second type, arranged for providing an electric signal at its terminal electrodes under the influence of a mechanical force being applied thereto. The mechanical detection element (41) and the sensor means (45) are in engagement with each other, in such a manner that mechanical forces exerted on the rail (17) during use are transferred to the sensor means (45) by the detection element (41). An alternating and/or pulsating electric signal is thereby continuously applied to the connecting electrodes of an element of the first type, and the electric signal provided at the terminal electrodes of an element of the second type is a measure for the mechanical forces exerted on the rail (17). In this manner the device has a "sel-monitoring" or so-called "fail safe" operation, as is required for many rail transport systems. The sensor elements of the first and the second type are preferably of a piezoelectric material.