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DENVER — Flaggers have returned to some crossings along the RTD A Line between Union Station and Denver International Airport.

In June, the flaggers were removed, but they have returned as RTD tries to figure out a software problem that was discovered on a number of rail lines in the area.

The flaggers have returned to the York, Steele, and Holly street crossings, a move that signifies RTD believes the automated gates are still not safe enough to operate without on-site supervision.

On Tuesday morning, the gates at the Holly Street crossing were closed for four minutes before a train arrived.

RTD is only saying its contractor, Denver Transit Partners, is doing some updating and testing of the crossing software.

But it has left residents dealing with horns blaring at all hours of the night. Residents were told the cities of Aurora and Denver would be allowed to apply for quiet zones once the flaggers were removed.

The return of the flaggers along the A Line could mean additional delays for the long-awaited G Line between Union Station and Wheat Ridge and Arvada.

That line was supposed to open in 2016 but has been delayed because of the gate timing issues on the A Line.