These aren't the only changes at Southwest. The Dallas airline previously announced that it was leaving behind another Southwest tradition, the open-boarding system it has used for more than 50 years.

United, JetBlue and Southwest said they are cooperating with the investigation. Amtrak services between Philadelphia and New York were shut down for five days following the accident.

These aren't the only changes at Southwest. The Dallas airline previously announced that it was leaving behind another Southwest tradition, the open-boarding system it has used for more than 50 years.