||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2014)|
|View toward the Transbay Tube|
|Location||San Francisco Bay, California, US|
|System||Bay Area Rapid Transit|
|Start||Embarcadero Station, San Francisco|
|End||West Oakland Station, Oakland|
|No. of stations||None|
|Opened||September 16, 1974|
|Owner||San Francisco Bay Area Rapid Transit District|
|Operator||San Francisco Bay Area Rapid Transit District|
|Line length||3.6 mi (5.8 km)|
|No. of tracks||2|
|Track gauge||5 ft 6 in (1,676 mm)
|Electrified||Third rail, 1000 V DC|
|Highest elevation||Sea level|
|Lowest elevation||135 ft (41 m) below sea level|
The Transbay Tube is an underwater rail tunnel which carries Bay Area Rapid Transit's four transbay lines under San Francisco Bay between the cities of San Francisco and Oakland in California. The tube is 3.6 miles (5.8 km) long; including the approaches from the nearest stations (one of which is underground), it totals 6 miles (9 km) in length. It has a maximum depth of 135 feet (41 m) below sea level.
The tube was constructed on land, transported to the site, then submerged and fastened to the bottom (mostly by packing the sides with sand and gravel). This immersed tube technique is in contrast to bored tunneling, where rock is removed to leave a passage.
Conception and construction
The idea of an underwater rail tunnel traversing San Francisco Bay was suggested by the San Francisco eccentric, Emperor Norton, in two proclamations that he issued in 1872. Official consideration to the idea was first given in October 1920 by Major General George Washington Goethals, the builder of the Panama Canal. The alignment of Goethals's proposed tube is almost exactly the same as BART's Transbay Tube. In 1947, a joint Army-Navy Commission recommended an underwater tube as a means of relieving automobile congestion on the then ten-year old San Francisco – Oakland Bay Bridge. 
Seismic studies commenced in 1959 and construction was started in 1965. The tube itself was finished in 1969. The tracks and electrification needed for the trains were finished in 1973 and the tube was opened to service in 1974. The tube is made of 57 individual sections that were built on land and towed out into the bay by a large barge. They were then positioned above where they were to sit and lowered into a trench packed with soft soil, mud and gravel for leveling along the bay's bottom. Once the sections were in place, bulkheads at each end of each of the sections were removed and a protective layer of sand and gravel was packed against the sides. It cost approximately $180 million in 1970.
During construction, the Transbay Tube was also used briefly as a shooting location for the end of George Lucas's film THX 1138. The final climb out to the daylight was actually filmed, with the camera rotated 90 degrees, in the incomplete (and decidedly horizontal) Transbay Tube before installation of the track supports, with Robert Duvall's character using exposed reinforcing bars as a ladder.
The western terminus of the tube directly connects to the downtown Market Street Subway near the Ferry Building, north of the Bay Bridge. The tube crosses under the bridge between the San Francisco Peninsula and Yerba Buena Island, and emerges in Oakland along 7th Street west of Interstate 880.
The 3.6 miles (5.8 km) Transbay Tube has required earthquake retrofitting, both on its exterior and interior. Earthquake research showed that the fill packed around the tube might be prone to soil liquefaction during an earthquake, which could allow the buoyant hollow tube to break loose from its anchorages. Retrofitting work required the fill to be compacted, to make it denser and less prone to liquefaction. On the interior of the tube, BART began a major retrofitting initiative in March 2013, which involved installing heavy steel plates at various locations inside the tube that most needed strengthening, to protect them from sideways movement in an earthquake. In order to complete this work during 2013, BART closed one of the two bores of the tube early on weeknights. The work, estimated to last approximately 14 months, was completed after only 8 months of construction.
In January 1979, an electrical fire occurred on a train as it was passing through the tube. One firefighter (Lt. William Elliott, 50, of the Oakland Fire Department) was killed in the effort to extinguish the blaze. Since then, safety regulations have been updated.
As a precaution, the tube was shut down along with the rest of the BART lines following significant earthquakes. The largest to date was the 1989 Loma Prieta earthquake, but the tube was found to be safe and reopened just six hours later. Many area highways were damaged, and with the Bay Bridge closed for a month due to a partial collapse, the Transbay Tube was the only directly passable way between San Francisco and Oakland.
In 2007, as BART celebrated the 50th anniversary of its creation, it announced its plans for the next 50 years. Its vision includes a new four-bore Transbay Tube beneath San Francisco Bay that would run parallel and south of the existing tunnel and emerge at the Transbay Transit Center to provide connecting service to Caltrain and the planned California High Speed Rail (HSR) system. The four-bore tunnel would provide two additional tunnels for BART and two tunnels for conventional/high-speed rail (the BART system and conventional US rail use different and incompatible rail gauges and operate under different sets of safety regulations). In the terminal, there would be 6 tracks: 4 for HSR and 2 for Caltrain.
- Emperor Norton, Proclamation of 15 June 1872 and Proclamation of 21 September 1872, both published in The Pacific Appeal newspaper.
- John Lumea, "An Earlier Tunnel Proclamation," The Emperor's Bridge Campaign.
- bart.gov History of the tube
- "Transbay Tube retrofit work wraps up early ending late night single tracking". Bay Area Rapid Transit District. December 2, 2013. Retrieved 2014-02-05.
- Cabanatuan, Michael (2007-06-22). "BART's New Vision: More, Bigger, Faster". San Francisco Chronicle. pp. A–1. Retrieved 2008-04-17.
- BART History
- BART History and Photos
- ASME National Historic Mechanical Engineering Landmark PDF - history and concept photos