The Twin Peaks Tunnel is a 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.
The eastern entrance to the tunnel is located at the west end of the Market Street subway, near the intersection of Market and Castro Streets, in the Castro neighborhood, and the western entrance is located at West Portal Avenue and Ulloa Street, in the West Portal neighborhood, named for the tunnel. There are three stations along the tunnel: West Portal at the western entrance, Forest Hill near the middle, and the now disused Eureka Valley near the eastern end.
History
Plans for a tunnel extending from Market Street under Twin Peaks were first presented at the Merchants' Association banquet in May 1909. A. W. Scott Jr. spoke on the need for the tunnel to open up the western part of San Francisco to development, as "40,000 San Franciscans lived across the bay or in San Mateo county because of the lack of proper transportation in this city." In April 1910, a committee, named the Twin Peaks Convention, was formed to plan the project, which would open the development of approximately one quarter of San Francisco's land area.
Competing proposals In July 1910, an architectural rendering was released of the eastern portal; the tunnel was initially envisioned as a two-story bore long, carrying road traffic on the upper deck and rail traffic on the lower deck. Market Street would be extended southwest in a straight line across private property to connect to the eastern portal, to be located at the intersection of 19th and Douglass. Initial estimates for the cost of the tunnel ranged from in 1909 to in 1910.
The Twin Peaks Tunnel and Improvement Convention released their report in August 1910 recommending a municipal bond to pay for the tunnel, whose costs would be recouped by the additional taxation of the land that would then be opened for development. Other options were considered and rejected, including a special assessment district or a long-term railroad lease.
The San Francisco Board of Supervisors approved a competing plan to build a longer tunnel under Twin Peaks in March 1912, continuing under Market to Valencia, which was seen as the foundation for a rapid transit system connecting the downtown Financial District with western San Francisco and the Peninsula. This action was taken on the recommendation of noted tunnel expert Bion J. Arnold, who had submitted two preliminary reports in 1912 and followed up with a final report in March 1913. The 1913 report studied several alternative alignments and configurations, recommending construction of both the Twin Peaks Tunnel and what would become the Sunset Tunnel.
Twin Peaks Tunnel proposal summary No. 1 No. 2 No. 3 No. 4 No. 53 3A4 4A5 5A 5B Notes AKA Schussler plan Proposed by Twin Peaks Tunnel Association High-level tunnel Includes transfer station near present-day Forest Hill station Combines aspects of Nos. 2 and 4, including transfer station at Forest Hill East Portal Market & Castro Market & Valencia Market & Castro 19th & Douglass Market & Castro 18th & Eureka Market & Valencia 19th & Douglass 18th & Eureka Route W on 17th to StanyanSSW between hills to Dewey Straight between portals West Portal Dewey Blvd Corbett & Dewey Laguna Honda near S end of Alms House tract Taraval & Dewey (near present-day West Portal station)
In the 1913 report, Arnold primarily considered proposals No. 2 and No. 5B, concluding "I can recommend unqualifiedly the construction of a Twin Peaks Rapid Transit tunnel at the earliest possible date. In so doing, there will be brought within 30 minutes' running time of the business district, approximately 10,000 acres of new territory, 75% of which is suitable for residence land, that has been practically useless heretofore by reason of lack of adequate transportation thereto." One of the key elements of proposal No. 5B was the creation of a new station along the tunnel, which is now present-day Forest Hill station (then called Laguna Honda station, at the intersection of Laguna Honda and Dewey Boulevard). The San Francisco Call speculated the presence of a station at Laguna Honda meant the Geary line could be extended through Golden Gate Park. A 1912 report, by Arnold, proposed that Laguna Honda would be a transfer point, enabling passengers to move from Third and Market to Ocean Beach within 25 minutes on an express car, assuming a Seventh Avenue surface line was built.
At a proposed in length, Arnold's proposed tunnel would not be suitable for road traffic for lack of adequate ventilation. He also believed the straight extension of Market to 19th and Douglass was impractical, as it would require extensive earth moving and exceed 3% maximum grade. It was on this basis that the Twin Peaks Association of Improvement in 1912 voiced their opposition to Arnold's plans. In February 1913 City Engineer M.M. O'Shaughnessy recommended terminating the tunnel at 17th and Market, as he thought the Market Street subway may need to be extended past Valencia, potentially to Third. O'Shaughnessy's report, endorsing Arnold's plan, was unanimously adopted by the Board of Supervisors in October 1913. With the addition of of "the best residence property on the peninsula within 15 minutes of the business center of the city", the Call estimated the population of San Francisco would add 100,000 within three years and 200,000 within five.
Carl Larsen, a prominent resident of the rural western side of San Francisco, recommended shortening the tunnel proposed by Arnold by moving the western portal to Laguna Honda station, but O'Shaughnessy stated the resulting grade would be too steep. Mayor James Rolph signed the Twin Peaks Tunnel Act in November 1913, rejecting the proposal for a shorter tunnel.
Construction and opening
The contract for tunnel construction was awarded on November 2, 1914, to the R.C. Storrie Company, for , and work began on the Twin Peaks Tunnel in December 1914, with an estimated three-year construction schedule for the twin-track bore. Construction was completed thirty-three months later. Funding for the tunnel came from special assessment districts established at the eastern and western ends; the undeveloped western area, approximately , was responsible for 85% of tunnel costs.
The eastern approach to the tunnel was built along a parcel purchased by the city approximately long and wide at the end of Market. This right-of-way was also later paved to connect Market with Portola. As originally constructed, the total length of the tunnel is (including approaches) and the highest point of the tunnel is near Forest Hill, originally Laguna Honda, station. Sloping down from the peak at Forest Hill, from west to east the tunnel has a maximum ascending grade of 1.15% and maximum descending of 3%. Excluding approaches, the tunnel is long.
The eastern portion (with the exception of the long Eureka Valley station) of the tunnel uses a double-tube subway structure, and the remainder adopts an arched tube profile. As designed, both sections provide of vertical clearance above the top of the rail; each tube in the double-tube section is wide, and the arched tube is wide at its maximum. The lunette space between the arched roof and flat ceiling of the tube was used as a duct to convey fresh air into the tunnel. One blower was mounted at the west end of the double-tube structure, and the other was at a ventilating shaft deep, located east of the western portal. Excavation of the ventilating shaft required pumping of water per day after a horizontal drift at the bottom of the shaft struck water-bearing sand.
Laguna Honda, West Portal, and Eureka Valley stations were constructed using cut-and-cover methods. Laguna Honda station is approximately below grade. The tunnel was excavated simultaneously from the open cuts at each end; initial excavation sank three drifts (one at each side along the springline, and one at the crown of the tunnel). A fourth drift was added at the centerline bottom of the tunnel to facilitate drainage of the ground at the western end. The two crews first "holed through" and connected their excavations near Laguna Honda station on April 5, 1917, from the east portal and from the west.
Several notable accidents occurred during the construction of the tunnel: A cave-in at the west portal trapped several men in April 1915, killing one. A fire broke out in November 1915. Four were killed and several injured while investigating a blasting charge that did not go off when planned in February 1917. A woman's house was nearly blown off its foundation by a blasting charge.
Dedication ceremonies were held at each portal on July 14, 1917. Mayor Rolph and several other dignitaries, including former Mayor P. H. McCarthy spoke at the east portal, beginning at 2:00 p.m.; after the remarks, hundreds of people walked through the tunnel to the west portal, and Judge E. P. Shortall was the first of the pedestrians to arrive. At the west portal, Mayor Rolph spoke again and a ceremonial silver spike was driven by O'Shaughnessy and Rolph to mark the start of construction of the Twin Peaks Railway. The inaugural trip through the tunnel occurred on December 31, 1917. The tunnel was opened to revenue service on February 3, 1918, and was the world's longest tunnel for street railway traffic at the time. Mayor Rolph served as the motorman for the first revenue trip, and most of the paying passengers were City Supervisors, wives, and invited guests. The trip through the tunnel took seven minutes.
Incorporation into Muni Metro
The tunnel was built sloping downwards at the east end, anticipating future expansion to a subway under Market Street per Arnold's plan. A technical report was commissioned by the city in 1960 to study possible rapid transit routes. This recommended that the initial line linking BART from a transbay tunnel to Daly City be routed through the Twin Peaks Tunnel. BART's 1961 plan did not include the tunnel connection on Market Street.
When the Market Street subway was built in the early 1970s, it was connected to the Twin Peaks Tunnel to be used by the K Ingleside, L Taraval and M Ocean View lines. These services were spared from conversion to trolleybus by virtue of their use of Twin Peaks Tunnel. The Eureka Valley station was closed and was functionally replaced by Castro Street Station. This combined subway line was the impetus for transitioning the system to light rail and served as the basis of Muni Metro.
The original eastern entrance to the tunnel in the middle of Market Street at Castro was removed and new entrances and ramps were constructed on the sides of the street further up the block. These were connected to track on 17th Street, and in December 1972 the K, L and M lines were diverted via the new East Portal ramps (and via 17th, Church Street and Duboce Avenue, removed from Upper Market) to allow construction of the Muni Metro subway under Upper Market to begin. These newer portals and ramps remained in use by all K-L-M service until 1980.