The two tunnel boring machines (TBMs) are named "Big Alma" and "Mom Chung" (for "Big" Alma Spreckels and Dr. Margaret "Mom" Chung, respectively). Preparations for tunnel boring began on June 12, 2012, with the start of excavation for the TBM launch box on Fourth Street between Bryant and Harrison. "Mom Chung" was delivered to San Francisco in April and May 2013, and in late July 2013, "Mom Chung" began digging the tunnel for southbound T Third trains. "Big Alma" began digging north in November 2013 at a slightly faster rate, , compared to the average of "Mom Chung".