The paper gives the first comprehensive description of the complete scheme of electrification of the Southern Railway. The system of electrification is 660-volt direct current with a top-running-contact conductor rail. The return circuit is by way of the running rails. Threephase electrical energy, generated or purchased at 11,000 volts, 25 c/s, and purchased at 33,000 volts, 50 c/s, is stepped down and converted to direct current for traction, by means of rotary-convertor and mercury-arc-rectifier substations. The paper includes some historical remarks, reasons for electrification, and results. It describes the power supply arrangements, the cable transmission system, the substations, the track equipment, and the electric vehicles. Particular attention has been given to the more modern equipment, and there is some amplification of detail on matters of unusual interest. Operating features that are of interest follow descriptions of the equipment to which they relate. Construction was stopped by the outbreak of war in September, 1939. The paper deals with developments previous to that time, but the electric locomotives completed subsequently are briefly described.