Bay Area Rapid Transit’s Board of Directors has approved construction of a 4.7-mile single-bore tunnel as part of the $4.8-billion second phase of a program to extend service to Santa Clara County’s Silicon Valley. BART claims the approach, the first use of a single-bore tunnel for a U.S. passenger rail project, will save 10 months of construction, compared with a twin-tunnel design, and minimize surface disruptions in downtown San Jose. Pending federal funding approval, work on the six-mile segment between the end of the extension’s first phase and the Norman Y. Mineta San Jose International Airport is set to get underway within the next two years.