WASHINGTON—United Airlines was fined $1.9 million Friday by the Department of Transportation (DOT) for violating federal rules on long tarmac delays and ordered to cease future similar violations. The department said between December 2015 and February, United allowed 25 flights to remain on the tarmac for lengthy periods without allowing passengers to deplane. United said Friday it remains “committed to fully meeting all DOT rules and will continue identifying and implementing improvements in how we manage difficult operating conditions.” The department said this is the largest fine of its kind ever imposed and impacted a total of 3,218 passengers. The Biden administration has pledged to take a tough line on airline consumer actions. United noted “ultimately only 25 out of nearly 8 million flights operated by United” and its partners warranted “enforcement action” over more than five years. Carriers are generally not allowed to keep planes on the tarmac for …