## Methods
### Interferometric Sensor Unit  
The process of constructing the measurement sensor can be divided into several steps. The main step in designing the interferometer sensor is correct positioning of its reference (\(L_{2}\)) arm and selecting the so-called \"resonant pad\" on which to place the measurement (\(L_{1}\)) arm. The reference arm material should be able to dampen any external influences applied to the reference optical fiber sufficiently. The measurement arm material should be of adequate quality to support attachment of this part of the sensor. In brief, the resonant pad needs to be placed on an elevated part of the trackside, and it should be able to transmit a vibration response from the surface to the measurement optical fiber (\(L_{1}\)). It should also allow the interferometer to be placed in a box that is resistant to external conditions. The dimensions of the waterproof plastic protective box were 40 \(\times\) 30 \(\times\) 12 cm (height, width, depth). It should also facilitate an I/O interface suitable for direct connection to feed single-mode telecommunications optical fibers (G.652).  
For the final construction of the damped part of the sensor, polystyrene foam was chosen because of its low thermal conductivity and ability to dampen higher frequencies. The tested minimum value for sufficient dampening of vibration transmission to the optical fiber was determined as 5 cm, mainly because of the sensitivity and frequency range of the interferometric sensor. Further, we tested the methodologies of installing measurement and reference fibers, waterproof boxes, splitters, fastening and housing materials, and optical fiber protections; please see our publications [43; 44; 45; 46; 47].  
For the final construction of the resonant pad of the sensor, we chose glass (soda-line glass) as an ideal material to contain the interferometer's measuring fiber. A series of laboratory measurements were carried out to verify the effect of the thickness and size of the glass (resonant pad) on the transmission of vibrations to the measuring (\(L_{1}\)) optical fiber. A thickness of 1 cm and a size of 50 \(\times\) 50 cm was selected (please see the publication [45]); these values showed a compromise with regard to sensitivity, frequency range, price, and practical installation. A photo of the fiber-optic interferometric sensor prototype is shown in Figure 3. The I/O interface of the sensor consists of the FC/APCconnectors (labeled red in Figure 3).  
Figure 4 below shows a longitudinal section of the sensory unit prototype. The symbols in Figure 4 indicate:  
1. Input/output interfaces (FC/APC connectors).
2. Protective plastic waterproof box.
3. Optical Coupler No. 1.
4. Conventional G.652 optical fiber.
5. Location of the measuring (\(L_{1}\)) arm of the interferometer.
6. Location of the reference (\(L_{2}\)) arm of the interferometer.
7. Resonant pad.
8. Optical Coupler No. 2.
9. Input/output interfaces (FC/APC connectors).  
The measuring arm is fixed directly (along with its entire length on the resonant pad). The dark grey part shows a part of the sensor, where a reference arm and both couplers are stored in the used polystyrene foam. The light grey part of the sensor shows a part of the storing of a measuring arm. The distance between the resonant pad with the fixed measuring arm and the dark grey part is 5 cm.  
Figure 5 shows the analyzed frequency characteristic of the interferometric sensor. The highest sensitivity was achieved in the range 2-100 Hz. Sensitivity levels in the range 2-100 Hz (in accordance with ISO 4866:1990, which states the range for rail transport as 1-80 Hz) were achieved by the design arrangement of the sensor described above.  
Figure 4: Longitudinal section of the sensory unit prototype.  
Figure 3: Sensor design with red labeled part included a I/O interface by FC/APC connectors.