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The second of two tunnel boring machines has begun work to construct two tunnels as part of the Central Subway project, San Francisco Municipal Transportation Agency officials (SFMTA) announced earlier this week.Like the first machine, the second is 350 feet long and weighs 750 tons. In the coming months, the two machines will travel north under 4th Street, Stockton Street and Columbus Avenue as part of the project to excavate and construct the city's first new subway line in decades, SFMTA officials said in a press release.When the Central Subway opens, the tunnels will enable the T Third Street Line trains to travel quickly beneath SoMa, Union Square and Chinatown, reducing travel times by more than a half hour along the busy corridor, they said.The machines will excavate and construct 1.5-mile-long tunnels at an average pace of 40 feet per day. Their pace will vary, depending on ground conditions and other factors; currently, the first machine is mining 20 hours a day, five days a week, SFMTA officials said. "The Central Subway and the tunnels … are essential to our vision and crucial to building and maintaining a reliable, modern public transportation system for San Francisco residents and visitors," said Mayor Edwin Lee.