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DENVER — The A Line commuter train to and from Denver International Airport could be shut down if RTD doesn’t come up with a plan to fix its warning systems, according to a letter from the Federal Railroad Administration.

The FRA wrote that it continues to have issues with the safety of the A Line warning system.

The federal agency gave RTD a Dec. 14 deadline for the transit agency to turn in a corrective action plan.

If not, the FRA has threatened to take away the train’s waiver that currently allows it to operate. The notice also covers the B Line, which runs between Union Station and Westminster.

“This level of noncompliance with the conditions of the waiver is unacceptable,” the FRA wrote in the letter to RTD.

The crossing gates have been an issue since the line between Union Station and the airport opened in 2016.

According to the government, in one week in August, inspectors found 63 instances where the gates closed earlier or lifted up later than federal standards require.

Crossing guards man the gates to make sure drivers don’t go around them.

Similar issues have been found on the yet-to-be-opened G Line from Union Station to Wheat Ridge and Arvada.

The notice comes months after the FRA said it has worked with RTD and Denver Transit Partners to make sure the crossing gates are safe and up to code on the A and B lines.

Denver Transit Partners operates the lines for RTD.

RTD responded with a statement saying it is, “Requiring DTP to work with the transit agency to develop a detailed corrective action plan acceptable to FRA.”

RTD also says the letter does not affect the safety or reliable service of the A and B lines.