The Transbay Tube is an underwater rail tunnel about four miles long connecting San Francisco and Oakland and transit leaders say it's reinforced before the next big disaster strikes.
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"The Transbay Tube now has a stronger steel lining," said BART General Manager Bob Powers. "It has a more powerful and robust pumping system that will protect itself against even stronger earthquakes, the type of earthquake that is projected to occur only once in 1,000 years."
Monday marks 50 years since BART started offering service through the Transbay Tube.
The Transbay Tube and BART's system withstood the shaking of the 1989 Loma Prieta earthquake and the system served the public on a 24-hour emergency schedule, less than 12 hours after the quake hit.
The Transbay Tube retrofit started in 2017. BART spokesperson Jim Allison said it also involved upgrading the pumping system.
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"One of the predictions is if there were some sort of earthquake that was very, very powerful, there could be small cracks that could create some minor flooding and so this pumping is designed to remove that flooding," Allison said.
This closes out BART's Earthquake Safety Program, which retrofitted more than 74 miles of the original track, 34 original stations and other infrastructure.
It cost nearly $1.5 billion and part of it was paid by a $980 million bond measure passed in 2004.