The Twin Peaks Tunnel in San Francisco, California begins service as the longest streetcar tunnel in the world at 11,920 feet (3,630 meters) long.

The Twin Peaks Tunnel is a 2.27-mile-long (3.65 km) light rail/streetcar tunnel in San Francisco, California. The tunnel runs under Twin Peaks and is used by the K Ingleside, L Taraval, M Ocean View and S Shuttle lines of the Muni Metro system.

San Francisco, officially the City and County of San Francisco, is a commercial, financial, and cultural center of Northern California. With a population of 827,526 residents as of 2024, San Francisco is the fourth-most populous city in the U.S. state of California and the 17th-most populous in the United States. San Francisco has a land area of 46.9 square miles at the upper end of the San Francisco Peninsula and is the fifth-most densely populated U.S. county. Among U.S. cities proper with over 250,000 residents, San Francisco is ranked first by per capita income and sixth by aggregate income as of 2023. San Francisco anchors the 13th-most populous metropolitan statistical area in the U.S., with almost 4.6 million residents in 2023. The larger San Jose–San Francisco–Oakland combined statistical area, the fifth-largest urban region in the U.S., had a 2023 estimated population of over nine million.

California is a state in the Western United States that lies on the Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares an international border with the Mexican state of Baja California to the south. With almost 40 million residents across an area of 163,696 square miles (423,970 km²), it is the largest state by population and third-largest by area.

A tram is an urban rail transit in which vehicles, whether individual railcars or multiple-unit trains, run on tramway tracks on urban public streets; some include segments on segregated right-of-way. The tramlines or tram networks operated as public transport are called tramways or simply trams/streetcars. Because of their close similarities, trams are commonly included in the wider term light rail, which also includes systems separated from other traffic.