Frustrated officials have summoned to Boston the chief executive of a South Korean company lagging far behind on a $190 million contract to build MBTA coaches, demanding answers for how the firm can avoid additional delays that would leave commuters relying on cramped, dated rail cars. Already nearly two years late, Hyundai Rotem appears to be in danger of falling further behind on its contract to build 75 double-decker cars, intended to replace some of the MBTA’s oldest single-level cars and expand the size and reliability of its commuter rail fleet.