In this study, a series of monotonically drained triaxial tests were conducted using the large-scale cylindrical triaxial apparatus. The effect of particle breakage and confining pressure on the mechanical behaviour of ballast were investigated. Based on the experimental findings, mathematical expressions for critical state stress ratio and specific volume are proposed to incorporate the evolution of particle breakage during monotonic shearing. An elasto-plastic constitutive model is formulated within the critical state soil mechanics framework in order to capture the stress-strain behaviour and degradation of ballast. The non-linear variation of shear strength envelope at a given range of confining pressures is taken into account. Mathematical expressions for shear hardening and stress-dilatancy relations are proposed. The constitutive model is calibrated against the results of large-scale laboratory tests and further validated using experimental results available from past independent studies. It is proved that the proposed method can promisingly predict the deformation of the ballast layer in a typical railway environment.