The earliest railway owned power plants provided 600 volts DC to the third rails and overhead wires. But the electric railways then transitioned to the more efficient central generation of the Commonwealth Edison utility. A large utility must transmit electricity over longer distances, which is most efficiently done at higher voltages. But the higher voltages must be transformed down to the usable voltages. And because AC is simpler to transform than DC, the Commonwealth Edison power plants must generate AC electricity.
And at substations within the neighborhoods throughout Chicago, the higher voltages would be transformed down to lower voltages for distribution throughout the neighborhoods. And smaller transformers, often attached to utility poles in alleys, would transform the electricity down the 120 volts normally used in homes, along with 240 volts for the larger appliances. And certain neighborhood substations would also transform electricity to 600 volts for public transportation. But one additional step was required at these substations, the conversion from AC to DC.
And in the early 1900's, when Chicago's streetcar companies and elevated railways transitioned to Commonwealth Edison electricity, the most practical technology for conversion from AC to DC was the rotary converter. Rotary converters resembled large electric motors, and with each revolution would "flip" the alternating current in its cycle, so it would become direct current flowing in the same direction. With its large moving parts, this technology seems primitive compared to today's rectifier installations. The Illinois Central Railroad's suburban electrification in 1926 was a pioneering installation using mercury arc rectifiers, for the conversion from AC to DC. And beginning in 1928, new substations constructed for Chicago's streetcar companies and elevated railways used mercury arc rectifiers.
At the time, Commonwealth Edison transmitted electricity in two different varieties, from the power plants from the substations. The most standard electricity transmitted is 12,000 volts at 60 cycles per second, or 60 hertz. But additional lines from the power plants transmitted 9,000 volts at 25 hertz. Or 20,000 volts at 25 hertz to some of the more distant substations. For industrial customers, the earliest heavy duty electric motors operated best at 25 hertz. And 25 hertz was also best for the rotary converters, supplying DC for electric railways. Although later rotary converters were designed to use 60 hertz electricity.
In addition, 600 volt "tie" lines connect between the substations, providing alternate paths for electricity.
In 1964, with CTA the only such customer remaining, Commonwealth Edison announced its intention to discontinue supplying the 25 hertz electricity. Commonwealth Edison and CTA agreed to share the costs for the modernization of the older substations, to enable complete standardization to 60 hertz.
The earlier substations had been constructed with sufficient space, for the rotary converters in addition to the transformers. At some locations, CTA eventually replaced some older substations with newer substations in smaller buildings.