SMART is adding a third rail car to several of its most popular round trips to add more room for the heaviest crowds of commuters. Sonoma-Marin Area Rail Transit officials said the addition of the third car earlier this month has proved popular, and the system may expand the larger service to other runs later. SMART trains usually travel in pairs, but the agency’s three-car train makes eight trips on the 43-mile line between Santa Rosa and San Rafael.