Federal regulators said Metro still isn't doing enough to emphasize safety during its 9-month effort to rebuild tracks and catch up on maintenance.
The Federal Transit Administration issued a report on Monday and directed Metro to take 12 steps to address safety concerns.
U.S. Transportation Secretary Anthony Foxx said the report "revealed significant safety concerns." He said the FTA "continues to provide vigorous oversight of Metrorail’s safety efforts in other areas of operation."
The agency noted that Metro didn't try to fix the problem area on the tracks where the recent derailment occurred because it needed the tracks to stay open while work was done nearby.
Regulators also found that Metro track inspectors are inadequately trained and that Metro doesn't give them enough time to perform their duties.
Metro also needs to coordinate with workers to adequately follow "established standards," including speed and single tracking restrictions, said FTA Acting Administrator Carolyn Flowers.
The FTA's report follows a nearly yearlong investigation of Metro's safety and maintenance practices following a derailment Aug. 6, 2015. It also contains regulators' preliminary findings about a derailment last month.
The investigation lasted between March and June, and focused on 10 sections of track from all six lines.