The first AC traction motors used 25 or 15 Hz power because motor designers had not discovered ways to decrease motor inductance enough for the motors to use regular 60 Hz AC power. The original East Cost electric railway lines used 25 Hz power until fairly recent times. Substations needed rotary converters to change 60 Hz power purchased from a power company to 25 Hz power for the trains. However most of the East Coast railroads like the New Haven had their own 25 Hz power plants to make electricity for their trains.

Most streetcars used 600 volt DC power because DC traction motors were perfected long before AC motors became available. The very first cars ran on about 400 volt power but as improved motors became available, the trolley voltage was gradually increased usually in 50 volt increments. A few streetcar lines were abandoned in the 1940's still using 550 volt DC power. With a large number of DC cars in operation, changing to AC power was not financially possible.

With 600 volt DC power, the rule of thumb was that a substation was required every 15 miles along the line. With 1200 volts DC substation spacing became 30 miles saving construction money and feeder wire cost. Sacramento Northern had some 1200 volt DC lines later changed to 1500 volts. Central California Traction Company was built as a 1200 volt DC line.

When a 1200 volt car is running with traction motors in series, it is the same as running on a 600 volt trolley line. So many 1200 volt interurban cars ran on regular 600 volt trolley car wires when in cities. They required only fairly simple changes to operate compressors and car lights from 600 volt lines. Many had devices to automatically change compressors and car lights when operating on 600 volt power.

The Washington, Baltimore, and Annapolis railroad was built in 1907 as an AC powered line. General Electric was so anxious to get into the AC interurban business that it signed a contract to re-equip the line for DC operation at its expense if the GE AC motor cars were not satisfactory. In 1909 the WB&A converted to 1200 volt DC operation mostly paid by GE.

A few interurban lines were built using 2400 volt DC power but they had many technical problems. Arcing from the electric third rail caused problems. Even a 1200 volt third rail line like Central California Traction Company had occasional problems with arcs between the third rail and truck frames. Unlike AC power that goes to zero volts 120 times per second, a DC arc will continue to burn continuously until power is shut off externally. That's why theaters used DC power on arc lamps in spotlights and movie projectors.

When control equipment was under the car floor as was typical of most interurban cars, operating voltage made very little difference. The controller in the platform was a remote control for equipment under the car car. However building a K-type controller for operation on the front platform of a car was a major problem. The Seashore Trolley Museum has a Maine interurban car with 1200 volt K-type controllers. They are real monsters and require lots of muscle to operate them. That's why the higher DC voltages were not used by streetcar lines.

Today many light rail lines use a nominal 750 volt DC power. There were no existing 600 volt cars or power systems. However light rail cars in cities like San Francisco where the old cars never went away still use existing 600 volt power systems.