Since the main reason of producing maximal mutational current in electrical railway load is not well known， so that the setting of startup component using current mutational value can't be calculated exactly and the line protective device often mis-operation. After analyzing lots of recorded data of Shanghai electrical railway load current， it is found that the magnetic inrush current of electrical train transformers is the main cause of maximal mutational current， and the maximum value of magnetic inrush current will almost be fourfold as much as the rating current of electrical train with maximal capacity. The gist and method for calculating maximal mutational current hereby is presented. The results show that the setting of mutational current calculated by the proposed method will not cause continually mis-starting of line protective devices. Suggestions of raising the sensitivity of startup component are proposed.