The test results raised the possibility that trains passing through that stretch could have had trouble receiving signals to stop or slow down. Officials stopped short of saying whether the sensors were broken, refusing to elaborate on the "anomalies" that testers found.
"Whether trains are operated in automatic or manual, these circuits are vital," said Debbie Hersman of the National Transportation Safety Boards, which is leading the investigation. "We're particularly interested in the speed commands that might be sent from that circuit when there's a train standing on that circuit."
Equipment along a 740-foot stretch failed to recognize a device that simulates the presence of a train during the tests, said Hersman. Five other stretches of track, or circuits, in the area of the crash near the Maryland state line showed no problems.
Hersman wouldn't give specifics on what the "anomalies" were or whether investigators think they were occurring before the crash, saying more tests were needed. Investigators planned to test the track with a six-car train Wednesday night.
An engineering professor who's studied transportation safety said that if sensors failed on the track, it could have contributed to Monday's crash, which killed nine people. He emphasized, though, that catastrophic crashes usually can't be blamed on a single factor.
"If the sensors didn't work properly, it deprived (the train operator) of very vital information," said Najm Meshkati, professor of engineering at the University of Southern California. "She was the last layer of defense."
The deadliest crash in Metrorail's 33-year history occurred when a train plowed into another that was stopped. The moving train was operating in automatic mode, which means it was primarily controlled by a computer, although there is evidence the operator tried to slow it down. Since the crash, trains have been manually controlled as a precaution against computer problems.
Hersman said inspectors found 300 feet to 400 feet of markings on the rails, indicating some emergency braking took place before the crash. Hersman also has said the emergency brake control on the moving train was found pushed down, though it's not clear how or when that happened. The operator of the oncoming train was among the dead.
Hersman said investigators hoped to interview the operator of the other train on Thursday, a day after his release from the hospital.
NTSB officials say their investigations can take more than a year.
The cars destroyed in the crash included some of the oldest in Metro's fleet. Federal officials wanted them phased out because of their tendency to compact in a crash, but Metro officials said the agency has lacked the money for replacements.
Meanwhile Wednesday, a union representing Metro transit workers demanded changes in safety procedures. Jackie Jeter, the union's president, said cars from the aging series involved in the crash should be placed in the middle of trains, rather than at the front or back because they are less stable.
The union also asked that operators be allowed to choose whether to use automatic mode, which is typically employed during rush hour. Jeter demanded that Metro officials not mandate speeds at which trains should travel, saying operators were being pushed to move too fast.
Metro spokesman Steven Taubenkibel said the agency was looking into the union's demands.
The union isn't the only one to raise concerns about running trains in automatic mode.
In Chicago's 'L' train system, operators never use an automatic system, said Robert Kelly, president of the city's transit workers union.
"If it were left to me, I would never have a train operate in automatic mode or autopilot," said Kelly, a former train operator.
"The problem with this job is complacency," he said, noting the risk of problems increases when operators aren't forced to stay alert.
Robert Jones, who retired from Metro after 18 years as a train operator and then came back in 2007 to work part-time, disagreed.
Operators have to keep an eye on hundreds of people at each train station platform during rush hours and prepare to stop for any safety concerns. They also open and close the doors, which requires them to look back from a window to make sure no one gets caught in the doors.
"You have to be aware all of the time," Jones said. "It's a job that requires your full attention."
Metro has said it needs $12 billion over the next 10 years to maintain services and replace aging equipment such as the older rail cars. The agency has long argued the federal government should contribute because its trains serve the capital, and many riders are federal workers.
Last year, Congress approved $1.5 billion in funds over 10 years, but with a condition that the District of Columbia, Maryland and Virginia match the amount. Now that all three jurisdictions served by Metro have agreed, Congress can move forward, officials said.
The region's congressional delegation introduced a measure Wednesday that would finalize plans to provide the funds.
Even with an infusion of cash to order new cars, Metro officials caution that it would still take about five years to get the equipment.
"It's not an off-the-shelf product," Metro spokeswoman Lisa Farbstein said. "It's something that's custom made."