BART Delays Possible As Central Subway Boring Machine Hollows Out Market Street Intersection

A tunnel-boring machine that has been slowly carving out the path for San Francisco’s new Central Subway system will pass under Market Street next week and could cause BART delays, Municipal Transportation Agency officials said.

Beginning Monday, the machine, nicknamed Mom Chung, will be crossing under Market Street near the Old Navy store at Fourth Street, SFMTA officials said.

The tunnels will be located about 10 feet below the BART tunnel, and will eventually form a new branch of the T-Third line that will connect the city’s South of Market neighborhood to Chinatown, with stops at Yerba Buena/Moscone and Union Square/Market Street, according to the SFMTA.

Mom Chung will be in operation 24 hours a day for the work below Market Street, and will then head north to Stockton Street.

The crossing under the BART tracks is expected to take up to six days, and could cause BART trains to run at slower speeds through the area. Information about BART delays will be available at or by calling 511.

“Working closely with BART and some of the top tunneling experts in the country, including our tunneling contractor, we will build this new tunnel under BART safely and soundly,” Muni director of transportation Ed Reiskin said in a statement.

“I have full confidence in our contractor and the many engineers and tunneling experts who are building the Central Subway under our city streets,” Reiskin said.

About 150 monitoring devices have been placed in the Powell Street station and neighboring buildings to allow SFMTA officials to receive live data feeds about tunneling progress.

More than 2,100 feet of tunnel has been created already underneath Fourth Street. Another tunnel-boring machine, Big Alma, is expected to cross under Market Street in January to create a second tunnel parallel to the first one for future trains running in the opposite direction, Muni officials said.

The $1.6 billion Central Subway is expected to open in 2019.

Updates on the project can be found online at

Dan McMenamin, Bay City News

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