Transbay Tube

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 (10 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.

All BART lines except the Richmond–Fremont line operate through the Transbay Tube, making it one of the busiest sections of the system in terms of passenger and train traffic.

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 today'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.