Metro rail cars that were sidelined in November after multiple incidents of cars separating while in service will stay off line, safety inspectors said Tuesday.

The Washington Metrorail Safety Commission said the 6000 series cars, a nearly 20-year-old model that made up about 15 percent of Metro’s active rail cars, will be out of service indefinitely because of problems with how parts were replaced during a recent overhaul. Safety inspectors said the repairs led to rail car separations on Oct. 9 and Nov. 24, both of which occurred on the Red Line.

Results of an investigation into both events were presented Tuesday to commissioners on the safety board, an independent agency Congress created in 2017 to oversee Metrorail safety after years of incidents and repeated safety failures.

“Metrorail did not have a comprehensive program in place, was not using the correct tools or procedures, and did not have oversight of the parts used in the overhaul process,” safety commission inspector Manuel Lopez told commissioners. “The 6000 series cars are out of service indefinitely.”

While both train separations occurred with passengers onboard, neither resulted in serious injuries.

In the October incident, two rail cars separated outside Union Station, stranding 108 passengers for nearly two hours. About six weeks later, rail cars on another Red Line train detached in a tunnel between the Wheaton and Glenmont stations in Montgomery County. Twelve passengers were evacuated.

Investigators found loose bolts and connections in the couplers, which are much like latches or hitches at the end of rail cars to hold the train together. The 6000 series is Metro’s sixth model of rail car in the agency’s 45 years of service. Built by Alstom, a French rail manufacturer, Metro said the cars’ couplers last were replaced in November 2019 as part of a Metro overhaul program. Metro officials have said couplers are typically replaced every eight years based on mileage.

Investigators said Tuesday that bolts were not properly torqued during that process, which led to the separations. Also pointing to the overhaul as the origin of problems was the discovery of nine 6000 series trains with couplers that had incorrect bolts and hardware, for which the manufacturer could not be responsible. They were found after Metro pulled all 184 of the cars out of service for inspections after one of the separations.

Additionally, the cause of a separation involving a 6000 series train in August 2018 outside the Silver Line’s McLean station was “similar” to last year’s incidents, Lopez said. Metro Chief Safety Officer Theresa M. Impastato has said a bolt that was the wrong size was to blame in that separation.

Lopez said details from Metro inspection reports show transit officials only checked specific fasteners in the couplers and not all of the fasteners. Investigators also learned that Metro’s overhaul process was incomplete when they checked the transit agency’s work with the manufacturer of the couplers. Maintenance workers also didn’t have the proper tools to overhaul the coupler, safety investigators said.

The commission ordered Metro to revise its inspection process and create a documented process to follow during overhauls. The 6000 series will remain grounded until all couplers go through an inspection.

“Metrorail is developing new procedures, procuring the required tools, and determining the best path to examine and properly rebuild the couplers,” Lopez said. “Metrorail may have a manufacturer do the work or may determine that it will do the work itself.”

Metro officials say the agency is deciding to proceed.

“At this time, all 184 6000-series rail cars remain out of service as we continue to evaluate the path forward in close collaboration with the coupler manufacturer,” Metro spokesman Ian Jannetta said.

During the commission’s investigations into the separations, other findings revealed poor coordination, communication and practices in how the transit agency responded to events. Some of the issues, including a lack of proper standards and procedures to preserve evidence after incidents are being addressed, the commission said.

In both events, safety commission inspector Bruce Walker said 911 calls made from the Rail Operations Control Center were not clear about the needed emergency response.

“This deficiency in the ROCC led to some delay and confusion,” safety commission inspector Richard David said.

He said Metro should develop a “standard response” for a train separation that coordinates with fire and rescue personnel what type of resources to bring, such as firetrucks, ambulances and specialists.

Communication and coordination problems during emergencies have confounded the transit system repeatedly over the past year. Metro officials have said they created a more urgent system of response, including staffing emergency management and safety department representatives in the Rail Operations Control Center at all hours to coordinate response.

Metro General Manager Paul J. Wiedefeld in April required employees to trigger a full emergency response when a passenger train, bus or MetroAccess car breaks down to ensure a full response to the scene of an incident.