The Sonoma-Marin commute rail line is exempted from having to give local design review boards its plans for stations and buildings under legislation that was signed by the governor on Friday. Such local oversight, while only advisory in nature, may have let any single city along the 70-mile line attempt to hold up construction, rail officials said. The Sonoma-Marin Area Rail Transit District already was exempt from local planning and zoning regulations, they said.