The economic distribution of the plant required for a railway electrification system is determined by the spacing of the substations. The paper deals theoretically with the factors influencing substation spacing, and contains formulae for calculating the ideal spacing in terms of the maximum allowable voltage-drop. Formulae are also included for determining the ideal spacing in terms of the maximum permissible overload-current setting of the track-feeder circuit-breakers to satisfy the condition that the current due to a short-circuit in the worst position must always be sufficient to trip the circuit-breakers.