The paper proposes a conceptual approach to economic efficiency substantiation of three-phase alternating current system of electric traction in comparison with one-phase alternating current system of electric traction. It is based on quasi-optimal values of indicators of electric traction system that are necessary for feasibility study and assessment of integral criterion of efficiency, for which, in turn, it is necessary to compare systems by reduced expenditures. Saving of reduced expenditures should also consider out-of-transport effect that is very important for system effect of interaction between railway transport and other enterprises.