Stepping from the South Station platform onto the 8:05 a.m. commuter train to Plymouth and Kingston - one of the weekend trains the MBTA plans to eliminate - Carol Payne of Rome, Ga., noticed something seemed missing: passengers. Turning to her husband, James, she asked, “How many riders does it take for them to run this?’’ The answer, according to the T, is a lot more than ride the train now. The train the Paynes rode to Plymouth for sightseeing Saturday could have been an exhibit for the T’s cost-cutting plans. Four of the train’s five passenger cars were closed off. At peak, the Paynes were two of only 13 passengers sitting in the more than 180 seats available in the sole open car.