Per the ikiw:

"In late 2017, a disagreement arose between VTA and BART over whether the tunnel should have a single bore or dual bores. VTA favored a single 45-foot-wide (14 m) bore, configured as a double-deck stack, with one track on the upper level and one on the lower level. VTA preferred a single bore to shorten the construction schedule and avoid cut and cover construction in Santa Clara Street for station sites. City officials believed cut and cover construction would be disruptive to streets and businesses, citing the construction of the Market Street Subway as evidence.[73][74][75] The single-bore design is newer, but less tested in the United States. However, BART preferred dual bores, as used elsewhere in its system, to cut construction cost and standardize the procedure for emergency evacuations. The twin bores would each be 20 feet (6.1 m) wide, and separated horizontally.[76][75] Local businesses, cities and VTA were lobbying for a single bore in 2018.[73] The design decision was postponed for three months;[77] in March, BART and VTA reached agreement on a single bore.[78][79] The $125 million contract for engineering the single bore tunnel was awarded to a joint venture bid placed by London-based Mott MacDonald and San Francisco-based PGH Wong Engineering.[80]

Initial construction and soil sampling began in January 2019.[81] In September,[82] it was announced that the project would be delayed three to four years, with revenue service to begin in 2029–2030.[14] On October 15, 2021, VTA announced that it would award construction contracts for this phase's stations in summer 2022.[83] In May 2022, the first major contract for the Downtown San Jose/Santa Clara extension was awarded to Kiewit Shea Traylor Joint Venture, a joint venture between Kiewit Corporation, J.F. Shea Co and Traylor Brothers, Inc. KSTJV will bore the 4-mile tunnel from Berryessa to Diridon, and construct the 1-mile above-ground track from Diridon to Santa Clara.[84][85]"