Twin Peaks Tunnel
| Company type | Infrastructure |
|---|---|
| Industry | Public transportation |
| Founded | 1917 |
| Headquarters | San Francisco, California , United States |
| Products | Rail tunnel for light rail and streetcar use |
The Twin Peaks Tunnel is a rail tunnel designed for light rail and streetcar use located in San Francisco, California. It passes beneath Twin Peaks and accommodates the K Ingleside, L Taraval, M Ocean View, and S Shuttle routes within the Muni Metro network.
Its eastern entrance lies at the western terminus of the Market Street subway, near the intersection of Market and Castro Streets in the Castro district. The western entrance is situated at West Portal Avenue and Ulloa Street within the West Portal neighborhood, which derives its name from the tunnel itself. Inside the tunnel are three stations: West Portal at the west end, Forest Hill positioned centrally, and the now-defunct Eureka Valley near the eastern side.
History
Initial proposals for a tunnel extending from Market Street beneath Twin Peaks emerged during a Merchants' Association banquet in May 1909. A. W. Scott Jr. highlighted the tunnel’s significance for unlocking development on San Francisco’s western side, pointing out that "40,000 San Franciscans lived across the bay or in San Mateo county due to inadequate city transportation." By April 1910, a Twin Peaks Convention committee was established to promote this initiative, aiming to open up roughly one-quarter of San Francisco’s land for development.
Competing proposals
In July 1910, an architectural concept depicted a two-level, 1.5-mile tunnel accommodating vehicular traffic on the upper level and rail transit below. The plan included extending Market Street southwest in a straight trajectory to an eastern portal located at 19th and Douglass Streets. Early cost projections varied from $1.5 million in 1909 to $3 million by 1910.
The Twin Peaks Tunnel and Improvement Convention advocated financing through municipal bonds that would be repaid via increased property taxes on newly accessible land. Alternative funding methods such as special assessment districts or long-term railroad leases were dismissed.
By March 1912, the San Francisco Board of Supervisors had endorsed a rival plan involving a longer tunnel beneath Twin Peaks, continuing under Market Street to Valencia. This concept was intended as a rapid transit corridor linking the Financial District with western San Francisco and the Peninsula. The approval followed assessments by tunnel specialist Bion J. Arnold, who evaluated various alignments and recommended constructing the Twin Peaks Tunnel alongside what later became known as the Sunset Tunnel.
| Proposal 1 | Proposal 2 | Proposal 3 | Proposal 4 | Proposal 5 | |||||
|---|---|---|---|---|---|---|---|---|---|
| 3 | 3A | 4 | 4A | 5 | 5A | 5B | |||
| Notes | Also known as Schussler plan | Proposed by Twin Peaks Tunnel Association | Elevated tunnel design | Incorporates transfer station near current Forest Hill station | Merges elements of Proposals 2 and 4, including Forest Hill transfer station | ||||
| East Portal | Market & Castro | Market & Valencia | Market & Castro | 19th & Douglass | Market & Castro | 18th & Eureka | Market & Valencia | 19th & Douglass | 18th & Eureka |
| Route | West along 17th to Stanyan South-southwest between hills to Dewey |
Direct line between portals | |||||||
| West Portal | Dewey Blvd | Corbett & Dewey | Laguna Honda near southern end of Alms House tract | Taraval & Dewey (near present-day West Portal station) | |||||
Arnold’s report from 1913 favored Proposals No. 2 and No. 5B, strongly advocating for rapid transit tunnel construction that would unlock approximately 10,000 acres—75% designated for residential development—within a half-hour commute to downtown. Proposal No. 5B included establishing a new station at Laguna Honda (now Forest Hill), facilitating a potential extension of the Geary line through Golden Gate Park and enabling express service to Ocean Beach within 25 minutes via a proposed surface route along Seventh Avenue.
With an approximate length of 16,000 feet, Arnold’s tunnel was considered unsuitable for vehicular traffic because of ventilation limitations. He also regarded the straight extension of Market Street to 19th and Douglass as impractical due to expensive earthworks and steep gradients. Opposition surfaced from the Twin Peaks Association of Improvement in May 1912.
Early in 1913, City Engineer M.M. O'Shaughnessy proposed ending the tunnel at 17th and Market Streets while anticipating future subway expansions beyond Valencia toward Third Street. His report supported Arnold’s recommendations and received unanimous approval from the Board of Supervisors in October that year. According to The San Francisco Call, this tunnel would open an additional 15,000 acres of prime residential land within a quarter-hour commute, potentially increasing the population by 100,000 within three years and doubling that number over five years.
Noted local Carl Larsen suggested shortening the tunnel by relocating its western portal to Laguna Honda station; however, O'Shaughnessy dismissed this idea due to concerns about excessive grades. Ultimately, Mayor James Rolph enacted the Twin Peaks Tunnel Act in November 1913, confirming plans for constructing the full-length tunnel without modification.
Construction and inauguration
On November 2, 1914, the contract for construction was granted to the R.C. Storrie Company for . Construction commenced in December of the same year, with an original timeline set for three years to complete the twin-track tunnel; however, the project concluded in just thirty-three months. Financing was secured through special assessment districts located at both tunnel ends, with the predominantly undeveloped western district, approximately , contributing 85% of the total expenses.
The eastern tunnel entrance is situated on a city-owned lot roughly long and wide at the terminus of Market Street, which was subsequently paved to link Market Street with Portola Avenue. The entire tunnel spans , including its approaches, with its highest elevation near Laguna Honda station. From Laguna Honda, the tunnel descends with maximum gradients of 3% traveling west to east and 1.15% heading east to west. Excluding the approach sections, the tunnel length measures .
The eastern segment (excluding Eureka Valley station) features a double-tube subway design, while the remainder utilizes an arched tube configuration. Both designs offer a vertical clearance of above the rails; each double tube measures wide, and the arched tube reaches a maximum width of . A lunette space between the arched ceiling and flat roof functions as an air duct system, ventilated by blowers positioned at the western end and a shaft located east of the western portal, extending deep underground. Excavation of this shaft demanded daily pumping of due to water-bearing sand encountered.
Stations at Laguna Honda, West Portal, and Eureka Valley were constructed using cut-and-cover techniques; Laguna Honda station lies approximately below ground level. Excavation proceeded simultaneously from open cuts at both portals, initially sinking three drifts and later a fourth along the centerline bottom near the west end for drainage purposes. The two excavation teams linked their tunnels close to Laguna Honda station on April 5, 1917, roughly from the eastern portal.
Significant construction events included:
- A fatal cave-in at the west portal in April 1915 resulting in one worker's death.
- A fire incident occurred in November 1915.
- In February 1917, a delayed explosion killed four workers investigating an unexploded charge.
- Blasting activities caused near displacement of a nearby residence.
Dedication ceremonies took place on July 14, 1917, at both tunnel portals featuring addresses from Mayor Rolph and former Mayor P. H. McCarthy. Several hundred attendees walked through the tunnel from east to west portals; a ceremonial silver spike commemorated the commencement of Twin Peaks Railway construction. The inaugural journey was conducted on December 31, 1917; revenue operations began February 3, 1918. At its opening, it held the distinction as the longest street railway tunnel worldwide. Mayor Rolph piloted the first revenue train, which lasted seven minutes and primarily transported City Supervisors along with their spouses and guests.
Integration into Muni Metro
The tunnel was engineered with a downward slope at its eastern end in anticipation of future expansion beneath Market Street as outlined in Arnold's plan. A city report from 1960 proposed routing the initial BART line through Twin Peaks Tunnel to connect a transbay tube with Daly City. However, BART's finalized 1961 proposal omitted this Market Street tunnel linkage.
During the early 1970s, Twin Peaks Tunnel was connected to the Market Street subway for service by K Ingleside, L Taraval, and M Ocean View lines, thereby preserving these routes from conversion to trolleybus operation due to their tunnel usage. The Eureka Valley station was closed and functionally replaced by Castro Street Station. This integrated subway system facilitated Muni’s shift toward light rail and laid groundwork for what became Muni Metro.
The original eastern portal at Market and Castro Streets was removed; new entrances and ramps were constructed along street sides further up the block to connect with tracks on 17th Street. In December 1972, K, L, and M lines were rerouted through these East Portal ramps to facilitate construction of the Muni Metro subway beneath Upper Market Street. These portals accommodated all K-L-M services until 1980.
Following completion of the Muni Metro subway segment between Van Ness station and Twin Peaks Tunnel in December 1980, weekday K, L, and M trains transitioned to Boeing LRVs running directly underground from Market Street subway into Twin Peaks Tunnel. Weekend operations initially continued using older PCC cars on surface tracks along Market Street; weekend subway closures persisted until September 19, 1982, when weekend K-L-M service fully converted to light rail with surface tracks permanently closed.
In summer 1983, surface tracks and the Castro portals constructed in 1972 reopened for San Francisco’s inaugural Historic Trolley Festival which eventually expanded to daily operation from May through October. Festival streetcars accessed Market Street through Twin Peaks Tunnel using these portals via Muni’s Metro Rail Center. The final Trolley Festival occurred in 1987; its successor line—the F Market—began service in 1995 using an alternate route via J Church extension to Balboa Park station for "pull-in" and "pull-out" movements, eliminating routine use of those portals. Light rail vehicles briefly utilized these portals during a two-week diversion in 1991 due to track maintenance near Castro station.
Originally designed with low platforms for streetcar boarding at Forest Hill and Eureka Valley stations, six new stations along Market Street subway incorporated high-level platforms for expedited boarding onto Boeing LRVs. West Portal station, once a surface stop outside the western tunnel entrance, was reconstructed as a high-platform facility inside the entrance. With closure of Eureka Valley station, Forest Hill remained low-platform until upgraded with high-level platforms by 1985.
COVID-19 effects and service resumption attempts
On March 30, 2020, all Muni Metro operations, including trains passing through the Twin Peaks Tunnel, were substituted with bus service as a response to the COVID-19 outbreak.
Light rail operations restarted on August 22 with adjusted routing designed to enhance reliability within the tunnel and at West Portal station. The K Ingleside and L Taraval lines were combined, operating between SF Zoo and Balboa Park without entering the tunnel, necessitating transfers at West Portal. Meanwhile, the M Ocean View and T Third Street lines ran between Sunnydale and Balboa Park. Extended S Shuttle services operated inside the tunnel. The transfer requirement at West Portal was met with opposition from disability rights groups.
Service was suspended again three days afterward due to a faulty overhead wire splice in the subway and a COVID-19 exposure among personnel in the control center, leading to a return to bus substitutions.
On May 15, 2021, rail operations resumed on the K Ingleside, T Third Street, and S Shuttle routes, with K Ingleside and T Third Street once more interlined. Some stations introduced new international-style wayfinding signs featuring compass directions. The M Ocean View line resumed service on August 14, whereas the L Taraval line continued as a bus shuttle because of ongoing construction. Initially, light rail service through the tunnel ended at 9 p.m., but starting October 2, Sunday hours extended to 10 p.m., and on other days until midnight to better coordinate with BART’s late-night schedule.
Rail replacement initiative
By approximately 2014, as the tunnel approached its centennial, Muni devised an extensive track renewal plan. This project involved replacing all rails and ties with new rails anchored to concrete pads, installing two pairs of crossover tracks, upgrading switches leading to unused eastern portals, structurally refurbishing the former Eureka Valley station area, replacing overhead wiring, among other enhancements. The objective was to eliminate the existing 25 mph speed limit and enable two-car train operation on K/T lines.
The construction contract was awarded on April 5, 2016. Although initially scheduled to begin in late 2016, the project encountered several postponements—to April 2017 and then mid-2017 for further technical evaluation. In June 2017, the contract was canceled due to disputes over timelines and budget increases; the project length expanded from 460 to 807 days with costs rising to $48 million. Subsequently, a new bidding process for a $35.5 million project commenced in late 2017.
Construction started on June 25, 2018, continuing until August 24. During this period, Muni Metro trains terminated at Castro station; surface-level K Ingleside service was linked with J Church, while L Taraval and M Ocean View lines were replaced by bus shuttles.
Widespread delays across the system during summer 2018 resulted from the tunnel closure combined with a shortage of qualified operators amid LRV4 vehicle introduction and labor contract complications. Work was temporarily halted following a fatal accident involving a signal technician employed by Shimmick Construction—the firm responsible for refurbishment—who had undisclosed prior safety violations. Muni received criticism for insufficient public notification during the closure and for lapses in vetting its contractors.
During construction activities, contaminated track ballast containing heavy metals was uncovered. To reduce expenses, Muni initially planned to reuse ballast without adequate washing or filtering procedures. By November 2020, this decision was recognized as likely contributing to track stability problems near Eureka Curve west of Castro station. Taking advantage of an unscheduled Muni Metro closure due to COVID-19 restrictions, ballast replacement work was carried out between November 2020 and February 2021.