Seven weeks after the MBTA pulled its new Orange Line trains from the tracks to deal with a mechanical issue, the first set returned to passenger service Tuesday as T officials hinted further disruptions may come as the updated fleet rolls out. One set of new cars began making normal trips Tuesday morning, and another will return “this month,” an MBTA spokesman told the News Service. “While the first new train set has returned to service, the MBTA will continue to closely monitor the new equipment and require vehicle engineering staff to be aboard for each trip to monitor performance,” MBTA General Manager Steve Poftak said in a Tuesday statement. “