System design, as its name implies, is the design of the electrification system for a particular route or area, and is not directly concerned with equipment design. The paper describes the steps by which the result is achieved, principally train performance, system loading, choice of electricity supplies, correlation with other undertakings and maintenance considerations, as well as the machine aids employed to ease the task of calculation and develop ment. The more precise design of equipment which has evolved over the years requires a more accurate design of the system as the margins are not now available to the same extent to cover unexpected events;further, increas ing economic pressures demand ever keener capital and operating costs. It is indicated how the design leads to the essential report. Reference is also made to likely future developments and procedures which should result in cheaper electrification.