New federal training requirements and a tight labor market have made it difficult for Maine’s three largest cities to staff municipal bus routes. Driver shortages are especially tough in Bangor, where more than one-quarter of positions are vacant, according to assistant city manager Courtney O’Donnell. The city will temporarily suspend Saturday service on the Community Connector starting June 18 to better staff bus routes during the work week. The Community Connector has 11 driver vacancies, O’Donnell said. “We are down so many folks that many of our drivers have to pick up doubles, and that’s really how we’ve gotten by up until this point,” she said.