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Interurbans
Alternating currents never made much progress with street railways and interurbans, except for power distribution to substations. Good AC series motors were not available until after 1900, and even then required 25 Hz current, not the usual 60 Hz. This meant special power stations and transmission systems. The Denver & Interurban Railway (Denver-Boulder) adopted 11 kV, 25 Hz around 1907 for country running, but used 550 V DC in cities. These heavy cars had a pantograph for the AC, and a trolley pole for the DC. The Westinghouse traction motors could operate on either AC or DC. The use of high-voltage AC completely eliminated feeders. The current was fed to the whole 33-mile line from one intermediate point through the catenary.

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