“We are still investigating,” Metro spokeswoman Caroline Lukas said Sunday. “We don’t know what the cause was yet.” Hundreds of passengers were stranded in the two incidents, and dozens of trains were forced to idle for up to 30 minutes on Saturday afternoon and again early Sunday until the system was restored.
Trains resumed normal operations Sunday, and officials said they did not anticipate any problems for the Monday morning commute.
The problems are the latest in a string of mishaps in recent weeks, including a stranded train near College Park and a derailment near West Hyattsville.
This weekend’s service disruptions occurred when a computer software program that provides controllers with a map of train locations went dark twice, Metro spokesman Dan Stessel said. As a precaution, train operators were told to move their trains to the nearest stations, where they remained until controllers could restart the system.
The first blackout occurred about 2 p.m. Saturday when Metrorail’s equipment in its operations center in Landover reported that the system for monitoring trains was not working. The second shutdown occurred about 12:30 a.m. Sunday and lasted until about 1 a.m.
Stessel said some trains were able to move before the software was fully restored. In such a situation, train operators can communicate by radio and can rely on track signals that are similar to traffic lights, Stessel said.
No similar problem appears to have been publicly reported in Metrorail’s 36 years of operation, but officials could not say whether this type of failure had occurred previously. Peter Benjamin, a former Metro board member, said that he could not recall a similar failure but that Metrorail officials would have to provide more information about the cause before anyone could give an accurate answer.
James Benton, chairman of the Tri-State Oversight Committee, which monitors the rail system, said Metro officials contacted the committee Saturday and are expected to submit a preliminary report this week.
The problems over the weekend differ from those at the root of the 2009 Red Line crash, which killed nine people, including a train operator. In that incident, the circuit system that feeds information to central controllers about train locations did not provide accurate information. As a result, a Red Line train was allowed to move forward because the system did not detect that there was a stopped train ahead on the tracks.
Nearly a year ago, as it continued an effort to improve safety and create backup systems, Metro moved its central operations center to Landover while maintaining an operations center in its downtown Washington headquarters.
That was part of the agency’s effort to provide a backup system should one operations center fail. But both centers rely on the same software system to track trains, and it is that system that failed over the weekend.
Even more crucial are the circuits on the tracks that provide data to the tracking software and to train operators, an expert said.
“If the circuit system is working properly, they don’t have to see the trains” at the central operations centers, said Ron Tolmei, a transit engineer who is familiar with the Metro system and is the former head of research and development at San Francisco’s Bay Area Rapid Transit system. “It does not rely on the computer saying ‘I can see it,’ ” he said.
After the 2009 Red Line crash, the National Transportation Safety Board concluded that Metro’s train protection system was inadequate. Some improvements have been made, but others are still being put in place.
Since the crash, Metrorail operators have been required to control all trains manually rather than relying on automatic systems that previously did much of the work.
The transit agency is replacing more than 1,500 track-circuit modules, and once that and other safety improvements are completed, automatic controls could resume.
On Monday, several federal officials, including the head of the NTSB, are expected at a previously scheduled news conference to discuss efforts to improve Metrorail safety.
The event, at the Landover operations center, is expected to focus on legislative changes to establish uniform federal safety regulations for every subway and light-rail system in the country, including large systems in Washington, New York, Boston and San Francisco.
Safety oversight of light-rail and subway systems has been delegated to 27 regional bodies controlled by states; for Metrorail it is the Tri-State Oversight Committee.
In a report this year, the NTSB said critical failings by Metrorail contributed to several rail incidents, including one in 2010 that killed two Metro technicians.
Metrorail’s disruptions came during a weekend of repair and maintenance on all five lines, which also caused slowdowns.
The track work had been scheduled for last weekend but was postponed because emergency repairs were required on a section of the Green Line after a July 6 derailment.
The derailment took place four days after a disorganized evacuation of a Green Line train that broke down outside the College Park station and stranded hundreds of passengers in sweltering rail cars.
Metro officials have said they will revise evacuation procedures as well as the policy for operations in extreme heat.
Lori Aratani, Marissa Evans, Joe Stephens and Lena Sun contributed to this report.