Granite Rock Company 10 is a preserved S100 class "Switcher" type steam locomotive built in August 1942 by the H.K. Porter for the United States Transportation Corps.
No. 10 was built in Pittsburgh, Pennsylvania, by the H.K. Porter in August 1942, for the United States Transportation Corps (USATC), originally No. 5001, it served in the Army depot in Tracy, California.
After World War II, in 1947, No. 5001 was sold to the Granite Rock Company of Watsonville, California and given the number 10. The locomotive worked at the company's A.R. Wilson quarry in Aromas, California until the 1950s, when it was replaced by diesel locomotives and taken out of service.
No. 10 stayed in Granite Rock's yards, until Bruce Woolpert the president and CEO, started a restoration effort in 1988, the work was undertaken at the shops of the California State Railroad Museum and was eventually donated to the museum in 1997. The restoration work included a new boiler built by the Dixon Boilerworks, in Los Angeles. The locomotive was fully restored to operating condition by May 1997, and became the principle motive power of the museum's Sacramento Southern Railroad excursion railroad.
On August 29, 1997, No. 10 was operated by an all-female crew to mark the anniversary of the ratification of the 19th. Amendment.
In 2010, No. 10 was loaned to the Niles Canyon Railway for their steam festival. The following year, No. 10 was taken out of service for extensive boiler repairs, before returning in April 11, 2015.
In July 2025, No. 10 was returned to service - after not operating since 2019 due to cylinder issues and the valve bushings being reworked.