**4. Conclusions**

The article combines theoretical simulations of the dynamic process of a rail response to the wheel load by a passing rail vehicle with precise tests on a real railway line, in which laser scanning with a sensor recording fast-changing processes was used.

By strengthening the transition zones, the negative effects of dynamic loads on the railway surface within the site of the change of the type of surface in front of the engineering site have been reduced by 14%.

By applying a gradual change in the elasticity of the rail support, the negative effects of dynamic loads on the railway surface within the place of change of the type of surface in front of the engineering object were reduced by 25%. Studies of concrete mixtures with different additives have already been undertaken which, in the laboratory scale, were used to make models of ballastless railway surface elements. The first results of the research indicate their possible applicability in practice.

Increasing the elasticity of the rail support within the transition zones before and after the object can be achieved by reducing the spacing of sleepers, better compaction of the ballast layer or strengthening of the upper layers of the subtrack, as well as by covering the track with one or more layers of transition plates [16,32].

It should be noted, however, that reducing the magnitude of the dynamic impact of a rail vehicle on the surface does not completely eliminate the phenomenon of the threshold effect, but only eliminates its negative impact on durability of the structure in zones where the type of surface changes [9].

The effects of the threshold phenomenon, in addition to vertical deformations of the rail, may also be other phenomena accompanying this effect, which may lead to an increase in track twisting and uneven wear of rails and damage to fastenings on both types of surfaces. Gaps may form under the sleepers, which threatens the stability of the structure. The threshold phenomenon has a negative impact not only on the railway surface, but also on the object that is exposed to excessive loads and vibrations.

Taking into account the above conclusions, areas exposed to the threshold phenomenon should be subjected to special supervision both in terms of current diagnostic activities and planned maintenance works.

In further research, in addition to the magnitude of displacements caused by the dynamic impact of a rail vehicle on the railway surface, it would also be necessary to determine the impact of these interactions on durability of the railway surface elements, such as sleeper rails and fasteners. It would certainly be valuable to determine the fatigue life of these elements in the zones of the threshold phenomenon and compare it to the nominal fatigue life for built-up elements outside the boundaries of transition zones. In addition, it would be worth identifying the processes taking place in the railway subtrack, i.e., the effects of the threshold phenomenon previously defined as "deep". The implementation of the above works would allow for a more complete knowledge of the impact of the threshold phenomenon on the condition of the railway surface and could be helpful in more effective planning of maintenance works within the transition zones in front of and behind engineering facilities.

Moreover, the load on a rail by a passing vehicle that was used in the article was movable, but constant as to the value adequately reflecting the weight of the vehicle. The comparative results are satisfactory. However, in future studies, the loading force will take into account the suspension structure of the vehicle, so its value will be determined by this suspension. Such a model was presented by authors Vahid Bokaeian, Mohammad Ali Rezvani, and Robert Arcos in [33].

**Author Contributions:** Conceptualization, T.L. and W.I.; methodology, T.L. and W.I.; software, T.L.; validation, W.I. and J.T.; formal analysis, T.L.; investigation, T.L.; writing—original draft preparation, T.L.; writing—review and editing, W.I. and T.R.; visualization, T.L. and D.P.; supervision, W.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Faculty of Civil Engineering and Geodesy of the Military University of Technology, Warsaw, Poland—grant UGB number 794.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Acknowledgments:** The authors would like to acknowledge: (1) the Authority of the Faculty of Civil Engineering and Geodezy of the Military University of Technology for providing administrative support during conducting our scientific work on this article, (2) the National Infrastructure Manager in Poland—PKP Polskie Linie Kolejowe S.A. Railway Lines Plant in Ostrów Wielkopolski (Wolno´sci 30, 63-400 Ostrów Wielkopolski) for enabling measurements to be carried out on active railway track, and (3) P.P.H. WObit E.K.J Ober s.c. (D ˛eborzyce 16, 62-045 Pniewy) company for providing a scanner for measurements.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
