Flaggers with orange vests and stop signs have returned to three crossings for the Regional Transportation District's A Line to Denver International Airport.
The Denver Post reports a "safety critical software problem" with gate crossings prompted the agency to place guards back on the line.
The flaggers were removed just over a month ago.
RTD says the guards are a "precautionary measure" while it figures out what's causing the gate software problem.
The Denver Post says the Federal Railroad Administration required guards to return to a number of crossings:
“A safety critical software problem was identified in the wireless activation system for all 13 crossings on the G-line and three crossings on the A-line and one crossing on the B-line,” FRA spokesman Marc Willis said Monday in a statement. “The FRA has required RTD use crossing attendants at those crossings until the issue is resolved.”