Insulated rail joints (IRJs) are safety critical components in the electrical signalling system of rail corridors. They are subjected to dynamic loads generated by heavy rollingstock – track system interaction and degrade faster than the other components of the rail track. Degraded IRJs diminish the reliability of the signalling system, thus posing a serious threat to the safety of rail operations. Therefore there is a pressing need for closely examining the failure mechanisms of the insulated material endposts inserted into the discontinuity in the rail at IRJs with a view to improving their service life, reliability and efficiency. Only limited literature are available examining different IRJ endpost materials, and these primarily focus on contact pressure and contact stress distributions in the vicinity of the endpost, disregarding the damage to the rail ends and endpost materials. In this paper, a detailed 3D finite element analysis (FEA) is carried out to quantify plastic deformation and material damage to endpost and railhead materials of IRJs due to a wheel load above the shakedown limit of rail steel. A modified Hertzian contact pressure distribution is considered in this simulation. A 5mm endpost thickness is considered at the discontinuity in the rail which is required to form the six-bolt IRJ. Three popular IRJ endpost materials are employed in this study, these being: fibreglass, polyhexamethylene adipamide and polytetrafluoroethylene. 2000 cycles of a 174kN dynamic wheel load (in pressure format over the wheel-rail contact patch) are applied on the top of the rail surface in the vicinity of the IRJ. Equivalent plastic deformations along with vertical and longitudinal plastic strains for unloaded conditions are presented. The strain plots depict damage of endpost materials and ratchetting failure of rail ends. The ratchetting failure modes follow the established trend of decay in ratchetting rate in successive wheel load cycles. Comparisons of strain and stress on the railhead surface and in the railhead sub-surface considering all three different endpost materials are put forward. Out of the three endpost materials, fibreglass is the optimal material considering the ratchetting mode of the railhead material damage.