The track deterioration rate is strongly influenced by the ballast behaviour under traffic. In order to restore the initial track geometry, different maintenance processes are performed, like tamping, dynamic stabilisation. A better understanding of the ballast behaviour under these operations on a portion of railway track is a key to optimize the process, to limit degradation and to propose some concept for a better homogeneous compaction. In this work, we focus on the study of tamping process over several sleepers by the numerical simulation using the Non-Smooth Contact Dynamics (NSCD) of three-dimensional Discrete Element Method (DEM) simulations improved by a Domain Decomposition Method and Parallel Computation.