Here is a Quick history of the North Shore Rail Road and yes it was quite heavily used

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The NSR was operated by John Martin and Eugene de Sabla, Jr., pioneers in the electric railroad business. The southern 23 miles (37 km) of line were modernized to allow operation of standard gauge electric passenger cars in addition to narrow gauge steam powered freight trains. Electric cars sometimes shared dual gauge tracks with the steam trains, while at other locations a separate track for the electric cars was constructed parallel to the narrow gauge route. The line was ultimately double tracked from Sausalito to San Anselmo except for the tunnel at Alto. A power house was built at Alto and power was also purchased at San Rafael. Direct current electrical power was transmitted to the trains at 600 volts by a third rail (which was actually a fourth rail on the dual gauge segments.)[22] Service started to Mill Valley on August 20, 1903, and to San Rafael on October 17, 1903. It was the first United States steam railroad electrified for operational efficiency rather than smoke abatement. The railroad established practices later used in Grand Central Terminal and the interborough subways of New York City.[23] The electric lines were expanded after 1907 as part of the Northwestern Pacific Railroa