The single-phase 50c/s system of railway electrification has developed extremely rapidly and has been adopted in recent years by France, Turkey, Portugal, the Belgian Congo and Great Britain. Of the many types of locomotive possible, the rectifier type is becoming more and more widely favoured, owing to its combining the best possible traction motor with the lightest form of convertor.The paper discusses various types of power circuit which have been or could be used on rectifier locomotives, including h.v. and l.v. control systems, types of on-load tap-changing circuit, buck-boost control systems, stepless control systems, bridge and bi-phase rectifier circuits, and methods of connecting motors. Different types of tap-changer, transformer and rectifier are discussed. The operation of the motor on current with ripples is compared with d.c. operation, and the many factors which influence the choice of the ripple current are analysed.The principal aim of the paper is to bring together the characteristics of transformers, tap-changers, rectifiers and motors, and to discuss the advantages and disadvantages of the various ways in which these items can be put together to form a complete equipment.No ideal solution is found, as the authors consider that the development potential is too great for any existing equipment to be regarded as final.