That move could be a signal that a settlement between the parties isn't likely.
Issues with the A Line's crossing gates required train operators to blare their horns at every crossing, annoying nearby residents.
The break from contractor Denver Transit Partners has divided some of RTD's board of directors.
That discovery came in an action plan the district submitted to federal regulators Friday and made public Monday.
The Denver Post reports a "safety critical software problem" at some crossings prompted the return of the guards.