Rail thermite (or Thermit®) welding is a valuable welding technology for the railway industry because of its simplicity, portability, and economy. Using the finite-element method, a heat transfer analysis is conducted in this study. Only the heat transfer by conduction was simulated explicitly in the model, the other thermally significant phenomena were included through the boundary and initial conditions. Despite these simplifications, the model can be used to analyse the thermal conditions during thermite welding with sufficient accuracy. The predicted temperature history, weld deposit, and HAZ profiles were in good agreement with the experimental measurements in laboratory welds. It is found that the weld gap is the most influential welding parameter for rail thermite welding. A wider-gap weld appears to be less sensitive to chance variations in the other welding parameters, and therefore fairly consistent thermal conditions can be obtained.