The Hetch Hetchy Railroad (HHRR) was a 68-mile (109 km) standard gauge Class III railroad constructed by the City of San Francisco to support the construction and expansion of the O'Shaughnessy Dam across Hetch Hetchy Valley. The mountainous terrain resulted in steep grades (over 4%) and extremely sharp curves (30 degrees, a 190-foot (58 m) radius), requiring trains to move slowly, at speeds of less than 8 miles per hour (13 km/h).