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The Santa Clara Valley Transportation Authority (VTA) and Bay Area Rapid Transit (BART) on Wednesday announced plans to conduct a peer review of single- and twin-bore tunnel methods.Slated to begin this month and last several weeks, the review will be conducted by public transit agencies that currently operate subway systems, VTA and BART officials said in a joint press release.One method calls for building a 45-foot-diameter single-bore tunnel that incorporates both trackway and stations within it. The other method involves building a twin-bore tunnel that includes two, 20-foot-diameter tunnels with stations and a wide center platform.The agencies are deciding which method they will use to construct a 5-mile tunnel as part of BART's Silicon Valley Phase II extension. That $4.7 billion, 6-mile project will run from San Jose, California, to an end-of-line station in Santa Clara. VTA serves as the designer, funding agency and builder of the BART Silicon Valley extension; BART will operate the system when it's built.