Passengers, almost all wearing face masks, board an American Airlines flight to Charlotte, on May 3, 2020, in New York City.Eleonore Sens | AFP | Getty ImagesPresident Joe Biden on Thursday signed an executive order requiring masks to be worn on airplanes, trains, buses and at airports as coronavirus infections continue to climb.The Trump administration declined to mandate masks for air travel and other modes of transportation, leaving it to private companies to set their own polices, though the Centers for Disease Control and Prevention has repeatedly recommended their use.That has left flight attendants and other employees to enforce the rules. Labor unions that pushed for a federal mask mandate cheered Biden’s order.”What a difference leadership makes! We welcome President Biden’s nationwide approach to crushing the virus and lifting us out of this pandemic,” Sara Nelson, president of the Association of Flight Attendants-CWA, the country’s largest flight attendant labor union, said in a statement. “Today’s, executive action on a mask mandate for interstate travel, including airports and planes, will provide much needed back up for Flight Attendants and aviation workers on the frontlines.Julie Hedrick, president of the Association of Professional Flight Attendants, which represents American Airlines’ cabin crews also applauded the move.”As passengers travel on different airlines and through various airports, they deserve to have clear expectations on what the rules are. We thank President Biden for seeing to this immediately,” she said in a statement.All major U.S. airlines require travelers to wear masks on board — a policy that extends to airports. Airline executives say the vast majority of customers follow the rule, but they have vowed to take a hard line against those who refuse. Through last week, airlines have barred more than 2,500 people from flying for refusing to wear face coverings. The FAA noted that some rare cases have even turned violent.The FAA this month warned it will crack down on unruly behavior and on travelers who don’t follow crew instructions, fining those travelers up to $35,000.Air travelers, including citizens, will also have to show a recent, negative Covid-19 test result before flying to the U.S. from abroad, Biden ordered, reiterating a CDC policy unveiled from last week. That rule takes effect Tuesday.Biden said travelers must self-quarantine upon arrival.