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Tamping process in railway engineering Mechanical model of the tamping unit‐ballast matrix interaction. Condition of the ballast bed is one of the most important parameters for safe and economical operation of railway systems. In order to define the optimum time for ballast bed cleaning or renewal as well as to improve ballast serviceability and track life cycle performance, better knowledge of the ballast bed condition, i.e. definition of the degree of ballast fouling is of crucial importance. As a core maintenance activity in ballasted track, the tamping process influences the economical service life of the track to a great extent and is essential in restoring the track geometry for safe train operations. During the tamping process, the tamping tines interact with the ballast matrix, transferring the displacement caused by the dynamic excitation overlapped with the extension in the hydraulic cylinder to the ballast, compacting it under the sleeper. This interaction is observed and measured in‐situ in several locations in Austria. Serving as a mean of comparison and confirmation with the conducted in‐situ measurements, a semi‐analytical model of the tamping unit – ballast matrix interaction has been developed. Conclusions regarding the differences in response and resistance to compaction of both new and fouled ballast material are made and presented in this paper. [ABSTRACT FROM AUTHOR]