The N Judah is a Muni Metro light rail line in San Francisco, California, so named as it runs along Judah Street for much of its length, named after railroad engineer Theodore Judah.[2] It links downtown San Francisco to the Cole Valley and Sunset neighborhoods. It is the busiest line in the Muni Metro system. It was one of San Francisco's streetcar lines, beginning operation in 1928,[3] and was converted to modern light-rail operation with the creation of the Muni Metro system in the late 1970s. While many streetcar lines were converted to bus lines after World War II, the N Judah remained a streetcar line due to its use of the Sunset Tunnel.

The line runs from the Caltrain depot in the Mission Bay district to Ocean Beach and the Great Highway in the Sunset District. From the Caltrain depot at Fourth and King Streets, it runs along King Street and the Embarcadero, passing by AT&T Park. It then enters the Market Street Subway, which it shares with the five other Muni Metro lines. It exits the tunnel at Church Street and, after a brief stretch along Duboce Avenue to Duboce Park, enters the older Sunset Tunnel. This tunnel serves to avoid a hill and contains no underground stations. From the western end of the tunnel, the route goes along Carl Street, pass UCSF-Parnassus Campus, towards Irving Street, until it turns onto 9th Avenue for one block and reaches Judah Street, which the N runs on for the rest of its route. On Judah between 9th Avenue and 19th Avenue the N runs on a right-of-way that is slightly raised above the surrounding street. There is a loop in the intersection at Judah, La Playa and Great Highway that the N uses to turn around.

The N Judah line stops at large stations for the downtown section of the route and at smaller stops on the rest of the line. Most of the smaller stops consist of nothing more than a sign on the side of a street designating a stop, while other stops are concrete "islands" in the middle of a street next to the tracks that provide access for wheelchairs. Muni bus routes provide service to all downtown stations and other systems with access to the stations are noted.

As with all Muni lines, service begins around 5 a.m. on weekdays, 6 a.m. on Saturdays, and 8 a.m. on Sundays and holidays. It operates at high frequencies, mainly between 7 to 12 minutes, and mostly utilizes two-car (150-foot (46 m)) trains during Muni Metro hours of operation. Late night service (after 12:55 a.m.) is provided by the N Owl diesel bus line. This line is generally the same as the daytime N Judah line, except it follows surface streets instead of going through the streetcar-only Market Street Subway and Sunset Tunnel. At the Ferry Portal at The Embarcadero and Folsom, it stays on The Embarcadero to Mission/Don Chee Way, then takes Steuart for one block and then turns onto Market Street, which it follows past all five underground stations served by the daytime N Judah line. It then takes Church, Hermann and Fillmore to get to Haight Street, where it bypasses the steep hill above the Sunset Tunnel, and serves the Lower Haight and Haight-Ashbury neighborhoods. It turns off Haight at Cole, and then rejoins the daytime N line at Carl.

On December 5, 2009, SFMTA eliminated the portion of the N Judah line between Embarcadero and 4th & King/Caltrain on weekends and holidays. (That portion is still served by the T Third Street line.) N Owl service was not affected by this change.[4] Weekend service to the Caltrain depot was restored in October 2011.

After concerns from riders of constant overcrowding of the trains on the N Judah line, Muni debuted an express bus route called the NX Judah Express on June 13, 2011.[5] Starting off as a pilot program, the NX (stylized as Nx) is intended to relieve overcrowding during rush hours every ten minutes. It follows the western end of the N Judah route from Ocean Beach to 19th Avenue, then operates nonstop from there to the Financial District where it stops at Bush and Montgomery Streets.