Short circuits applied close to substations cause large transient peak fault currents, but remote faults are difficult to detect because the fault current may be less than a normal train load. It is important to be able to predict the form of both closeup and remote fault currents for design of new protection systems and to assess the suitability of particular protection arrangements for specific railways sites. The model of the substation rectifier is an essential part of any means of predicting the current profiles. A detailed rectifier model based on a piecewise linear digital simulation, particularly suitable for prediction of fault current profiles owing to closeup faults is presented. The accuracy is demonstrated by comparison with a case study of a practically measured fault current profile. It is shown that commonly used models based on simpler equivalent circuits can not match the accuracy of the detailed model. It is also shown that a simple DC power source model of a rectifier is appropriate for remote fault calculations because the profile is not significantly affected by the switching action of the rectifier. The validity of the simpler model is demonstrated by comparing the simple model with the more detailed one.