Mom Chung -- a 350-foot-long, 750-ton tunnel-boring machine -- began excavating a new subway tunnel in San Francisco. The machine will spend about 10 months digging a 1.5-mile tunnel as part of the $1.6 billion Central Subway project. A second TBM will bore a parallel tunnel. The project will add 1.7 miles to the Third Street light-rail system.

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