**5. Conclusions**

This article is a continuation of the research described in [13], in which the phenomena occurring in the zones of changes in the technology of the railway surface were analyzed. In the transition zones from the ballast surface to the ballastless surface of the railway line, there are jumping changes in the strength parameters of the materials from which the elements of the surface are made. This adversely affects the dynamic interactions of the track–rail vehicle system. This phenomenon, called the threshold effect, was considered in [13], where reinforcements of materials on the ballast surface side were proposed, partially reducing the adverse effects of the threshold phenomenon. This paper presents

how it is possible to influence the gradual change of strength parameters of materials on the ballastless side using concrete with various, gradually changing additives for the construction of pavement elements. As a consequence, the strength parameters in the transition zone, from ballast to ballastless surface at the entrance to the engineering facility and from ballastless to ballast surface when leaving the zone of the engineering facility, can be constructed in such a way that the change in the strength parameters of the materials used is not incremental, but rather changes continuously. It has been shown that this would be possible by using various concrete additives so that in a zone of appropriate length, the strength properties of the pavement elements change as continuously as possible.

The next step in the test cycle could be the use of an experimentally proven computational model [13], which would exploit the possibilities of affecting the ballast part of the surface in the transition zone [13] and the ballast-free part, discussed in this article, so as to propose design solutions in which the dynamic vehicle-track interactions will be limited as much as possible.

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

**Funding:** This research was funded by the 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 the Authority of the Faculty of Civil Engineering and Geodesy of the Military University of Technology for providing administrative support during conducting our scientific work on this article.

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