The paper presents a technique for calculating the current/time profile for a remote short circuit fault at a DC rapid transit railway substation. The profile is largely determined by steel rail impedance, which is shown to vary with time within the profile. An inverse Fourier transform, employing a novel antialiasing method is applied to obtain the profile from frequency based impedance data. The technique is demonstrated to be accurate to within 10% with a comparison between a calculated profile and practically measured results obtained on a light rail system. The impedance data required in this technique can be obtained by either measurement or calculation and scaling of standard values using the method described in a companion paper.