In order to understand the effect of particle breakage on the mechanical behaviour of railway ballast, a series of monotonically loaded drained triaxial tests was conducted using the large-scale cylindrical triaxial apparatus. The critical state behaviour of the ballast was investigated. A non-linear critical state envelope is determined in the q-p' and v-lnp' planes. 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 based on the critical state soil mechanics framework is presented capturing the stress-strain behaviour and degradation of ballast. The constitutive model is able to predict the monotonic shear behaviour of ballast observed in the laboratory tests. The model is validated using experimental results available from past independent studies.