The size and shape of particles influence how effectively coarse angular aggregates of ballast interact. The aim of this study was to improve the characterisation of ballast particles using a threedimensional (3D) imaging method. Various size and shape indices, such as elongation ratio, sphericity and roundness, were determined from the scanned 3D images. A modified index called ‘ellipsoidness’ was proposed to capture adequately the shape of the 3D particles. Variation of these indices with particle size was studied. Comparison of the 3D true sphericity and the corresponding two-dimensional sphericity indicated that the latter would underestimate sphericity. A modified approach for transforming particle size distribution to constriction size distribution is proposed by capturing the size and shape effects of particles.