Initially it was envisaged the single-bore tunnel, currently in design and engineering, would be a two-level construction but contractor Kiewit Shea Traylor (KST) joint venture has now proposed that the two tracks run parallel. The configuration is expected to be cheaper to construct, easier to maintain, make access more efficient and provide better ventilation.

KST was awarded the US$235m progressive design build contract in May this year.

Contract Package (CP2) will be carried out in stages, with Stage 1 activities including investigation of innovations, engineering and design, open book cost estimates, and the work schedule. Stage 2 will be the major construction phase, including boring the tunnel under downtown San José.  

Five miles of the six-mile extension – the largest single public infrastructure project ever constructed in Santa Clara County – will be a large diameter single-bore tunnel. The project will take the BART service from Berryessa/North San José through downtown San José to the City of Santa Clara. It includes three stations with underground platforms (28th Street/Little Portugal, Downtown San José, and Diridon), one ground-level station (Santa Clara), a train maintenance and storage facility at Newhall Yard, and additional facilities.

Main construction works are expected to start in autumn next year and the extension will be operational by 2030.

Phase I began in 2012 and the service opened in June 2020.