Owing to the small number of grains in the vertical direction of the ballast layer of a railway track, a discrete/continuum double-layer model is proposed: the upper layer (the ballast) is modelled by a discrete 3D-lattice of interacting grains whereas the infinite underlayer is a continuum linear elastic medium. The problem under consideration is the dynamic behaviour of this double-layer system under the action of a moving load. A parametric study concerning the underlayer stiffness, the load velocity and the grain size is conducted. A comparison with the continuum modelling is provided for realistic moving loads.