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