TWIN PEAKS TUNNEL

The 'Twin Peaks Tunnel' is a 2-mile (3.2 km) long streetcar tunnel in San Francisco, California, United States running under Twin Peaks. When it was opened on February 3, 1918, it was one of the longest railway tunnels in the world (Paris Métro's line 1, opened in 1900, beat it by a few miles, as did some of the London Underground lines). It was the longest U.S. railway tunnel west of New York City. It is still the world's longest streetcar tunnel. It runs from the intersection of Market Street at Castro Street in the east to the West Portal neighborhood in the west. There are two stations along the tunnel, Forest Hill Station, near the western end, and the now disused Eureka Station, near the eastern end.
When the Muni Metro system and Market Street Subway was built, it was connected to the Twin Peaks Tunnel to be used by the K Ingleside, L Taraval and M Ocean View lines. The Eureka Station was closed, and the Metro lines stop at the nearby Castro Street Station instead. The original eastern entrance to the tunnel in the middle of Market Street at Castro was removed and new ones were placed on the side of the street further up the block, though no Metro or streetcar lines use them in regular service (they are occasionally used in non-revenue service such as rerouting trains around construction projects). Instead, trains continue directly from the Market Street Subway into the tunnel, without going above ground. A new station with high platforms, West Portal Station, was also built for the Metro lines at the western end of the Twin Peaks Tunnel.