## 2 State-of-the-Art  
The authors of [11] provided interesting results describing bridge statics and a dynamic vibration monitoring system. In that study, two different types of cameras were used to monitor the response of a bridge to a passing train. The acquired images were analyzed using three different image processing techniques (pattern matching, edge detection, and digital image correlation), and the results were compared to reference measurements obtained by a laser interferometer providing single point measurements. The authors of [12] made use of an in-fiber Fabry-Perot interferometer with fiber Bragg grating mirrors (FBG-FPI). The authors suggested that this type of sensor could be used to monitor a wide range of vibration frequencies. One of the possible applications is in monitoring seismic responses. The work in [13] compared and assessed the suitability of two methods of geodetic measurements (interferometric measurement and tachymetry measurement). The tachymetry measurements were conducted using a Leica TC2002. An IBIS microwave coherent radar was used for the interferometry method. The authors monitored dynamic changes caused by passing trains. The experimental results could be used to determine the changes in the geometric condition of buildings and engineering structures. The authors of [14; 15; 16] focused on the use of a fiber-optic interferometer in motor vehicle traffic or for perimetric applications. A sensor on the Mach-Zehnder interferometer was constructed to detect vibration and acoustic responses caused by humans and vehicles moving around the sensor. The authors described the possible use of this sensor to monitor traffic density, vibrations from roadways, and noise in cities dynamically. Another publication described a strain meter based on a laser interferometer. This sensor provided a new method for observing low-frequency ground motion on seismic, geodetic, and intermediate timescales, as described by the authors of [17]. A study of the geophysical response of fiber-optic acoustic sensors in laboratory-scale experiments was described in [18]. The authors analyzed acoustic data recorded simultaneously by a fiber-optic interferometer (Michelson type) and conventional three-component accelerometers. Another use of interferometric sensors was described in [19; 20]. The authors of [19] reflected on the use of an acoustic fiber-optic sensor to monitor railway infrastructure from a long distance on the basis of an interferometric connection. The work in [20] described the implementation of optical sensors with a Fabry-Perot interferometer during the production of pre-stressed, pre-fabricated concrete, and steel sleepers. These sensors can be used to calculate the number of axles, as well as the direction and speed of a train with additional sensors. In the patent described in [21], an interferometric sensor was used to detect railway vehicles and calculate their number of axles, i.e., an axle counter. The principle of detection used in this patent is based on scanning vibration responses as trains pass. To apply this sensor, its sensing element must be placed in direct physical contact with the rails. The reference element, including its evaluation component, is then placed away from the rails. In patent [22], the basic principle of a railway vehicle axle counter based on a fiber-optic interferometer is described. The sensing element of the optical fiber of the interferometer is physically attached to the rail, and a multimode optical fiber is included in the railhead. A change in the geometry of the optical fiber because of the pressure exerted on the fiber by a passing railway vehicle changes the speckle images of the measuring fiber. The work in [23] described the use of a Mach-Zehnder fiber-optic interferometer to monitor car traffic, for detecting vehicles, and measuring their speed. A Mach-Zehnder interferometer for rail transport was described in [24], which analyzed the use of a three-armed Mach-Zehnder interferometer in rail traffic as a traffic density detector.