Alto Powerhouse

December, 2009

Undated photo of the Alto Powerhouse – click to enlarge

From 1890 to 1902, the North Shore RR served Mill Valley via a branch line. The main line went from Sausalito along Richardson Bay and through the Alto tunnel to Corte Madera, San Anselmo and beyond. The trains to Mill Valley had steam locomotives that burned wood until 1900 when they switched to oil. In 1903, the North Shore RR electrified the line from Sausalito to Mill Valley. The view from Homestead Valley looking east became dominated by the Alto Powerhouse which provided the electricity. It was located on the side of a hill on the far side of Richardson Bay. A railroad siding from the main line had been installed to allow delivery of construction materials as well as fuel oil for its boilers.

The large brick building, 61 ft. by 171 ft., was divided into an engine room, a battery room and a boiler room. Adjacent to the engine room was a tower for the high voltage transmission line. Transformers stepped down three-phase 50,000 volt alternating current to 4500 volts. This powered motor-generator sets producing 600 volt direct current which was fed to the third rail along the railroad tracks. Steam generators in the boiler room could provide 600 volt direct current in case of a failure in the high voltage transmission line.

The electricity was generated at the Colgate hydroelectric powerhouse on the north fork of the Yuba River. The 150-mile high voltage transmission line passed through Woodland, Napa, Petaluma and San Rafael. It was the longest such line in the world. The electrified railroad was the first in California to involve a third rail.

By 1930, electrification of the railroad had been extended to Corte Madera, San Anselmo, San Rafael and Fairfax. There were power stations in Sausalito, Larkspur, San Anselmo and San Rafael. But Alto was the only installation that required an operator; the others only had batteries which switched on automatically at the approach of a train. The last Mill Valley passenger train ran on September 30, 1940.

The Alto Powerhouse was demolished in the 1960s. On the site today is a PG&E transformer substation on Roque Moraes Drive opposite Longfellow Road.