Developing the railway transport sector is a challenging scientific, economic and social research topic starting with ensuring human security. The main topic that should be developed in that sense is the ballast stability and dynamical behaviour under external loading and environmental changes. This paper investigates the effect of particle size distribution and normal pressure on the mechanical response of a ballast bed. Grading curves of ballast layers with different sizes are illustrated to discuss their strength behaviour under various strains to deduce the significant effect on the direct shear performance of the ballast layer. Direct shear tests with different Particle Size Distribution (PSD) were reproduced using the Discrete Element Method (DEM). It is noticed that when the number of small-sized ballast increases, the shear strength and the friction angle increase to varying degrees under different normal pressures, with an average increase of 27 % and 8 %, respectively. When the number of large-sized ballast decreases, the shear strength and the friction angle decrease to varying degrees under different normal pressures, with an average decrease of 6 % and 3 %, respectively.