New issues identified with the automated train control system on Phase 2 of the Silver Line extension could pose a “potentially significant safety concern” for riders, Metro said in a letter to officials managing the project.

The problems, outlined in a letter sent this month to the Metropolitan Washington Airports Authority by Neil Nott, who oversees the project for Metro, involve the track circuit design of the automatic train control system (ATC).

Nott said it appears that the project contractor, Capital Rail Constructors, did not meet design criteria for the system.

“[The Washington Metropolitan Area Transit Authority] has significant concerns that the ATC system . . . does not comply with the requirements . . . and industry standards for circuit design, particularly as intended to facilitate broken rail detection and avoid the potential for loss of train detection or derailment,” Nott said in the letter, a copy of which was obtained by The Washington Post.

“WMATA requests that MWAA require CRC to perform a comprehensive review of the track circuit design requirements and perform and submit a comprehensive safety assessment supported by appropriate analyses and calculations to demonstrate that the ATC System is safe.”

Nott wrote that the spacing of the cross-bonds, large cables that enable current to flow from one track to another, have been installed in a way that could make it difficult for the system to detect cracked rails, hamper its ability to detect when trains are present or put trains at risk for derailment.

A Metro spokesman declined to comment beyond the letter.

The automatic train control system is critical to safe operation of the rail system. The system relies on track circuits to transmit speed commands so that trains maintain a safe distance from each other and stop when the track ahead is occupied. A failure of the system can have catastrophic consequences as was the case in the 2009 Red Line crash near Fort Totten station. In that instance, a track circuit failed to detect the presence of a train and allowed a second train to move forward at full speed. Nine people were killed and dozens of others were injured.

A spokeswoman for MWAA, which is overseeing construction of the rail extension, said they were aware of Metro’s concerns and were waiting for CRC to complete its investigation into the issue.

“We have forwarded the letter to CRC and asked them to do an investigation,” said Marcia McAllister, a spokeswoman for the rail project. “We are awaiting their response.”

A CRC spokesman said the company remains committed to delivering a “safe, reliable system for the region’s riders,” and noted that both MWAA and Metro approved the design.

“We are reviewing WMATA’s letter with the engineer of record and the ATC supplier,” said Keith Couch, project director for CRC. “We have installed the system, and are testing the system, in accordance with the design that was engineered by the engineer of record, Parsons Transportation Group, and reviewed and approved by both MWAA and WMATA. To date, testing has not revealed a safety issue related to this matter.”

Phase 2 of the Silver Line extension has six stations, including one at Dulles International Airport, and will extend Metro service into Loudoun County. The first phase of the project opened in July 2014 with four stations in Tysons and one in Reston.

Originally scheduled to open this year, the project is more than two years behind schedule, though officials think it will be ready for passenger service in 2021. Project officials said it is too early to know whether this latest development will have an impact on those plans.

This problem is separate from the litany of other issues that have plagued the project since construction began on the second phase of the $5.8 billion extension in 2014, including cracks in concrete structures, defective rail ties, a rail-yard platform that had to be ripped out because it was built to the wrong dimensions, and a Justice Department investigation involving a subcontractor.

However, this latest issue is likely to renew concerns about MWAA’s oversight of the rail project — and why such a potentially significant issue was not identified earlier. McAllister said project officials don’t know why the problem wasn’t found sooner.

In March, a report by Metro Inspector General Geoffrey A. Cherrington said that MWAA and the project’s contractors have failed to thoroughly investigate the core cause of problems that have surfaced on the rail project, including malfunctioning surge arresters and safety-critical portions of track that experienced “an abnormally high number of failures” and that the lack of follow-through has allowed issues to go unresolved for months, even years.