(Robert Thomson/The Washington Post)

The failure of emergency intercoms on Metro trains, which the transit agency confirmed  Wednesday, occurred on train setups that became more common after the Red Line crash in 2009.

Emergency intercoms did not work when cars from the 6000 series, the newest in Metro’s fleet, were followed by older cars from the 1000 series or the 4000 series. Riders on these older cars would not have been able to contact train operators at the front of the train, an issue that cropped up this week after  a fight aboard a Red Line train.

After the Red Line crash that killed nine people in 2009, Metro moved cars from the 1000 series to the middle of train sets. As a result, “it became a much more common occurrence” to have trains from the 1000 series trailing cars from the 6000 series, Metro spokesman Dan Stessel said  Wednesday.

Metro noted in a news release issued in 2009 that mixing trains in this fashion meant “that sometimes the intercoms used by customers to communicate with the train operators also do not always function, and officials are seeking to identify a fix to that situation.”

The transit agency is conducting a review that will determine, among other things, whether the precise problem could have been identified sooner, Stessel said.

“I would assume we’re talking about the same issue,” he said  Wednesday. “This issue has existed for as long as [the] 6000 series car has been on the property.”

Intercoms are  in every rail car and are meant to let riders contact the train operator to report suspicious behavior or emergencies. They are particularly vital because riders who need to report sick passengers or other issues have no other way to contact the operator.

“Even if they’re in a station and their cellphone is working, they’re not going to use their cellphone to call the train operator,” Mort Downey, a Metro board member, said  Wednesday. “Literally the only way they could address a message to that train operator is via the intercom.”

The results of the transit agency’s review will be presented at a meeting of the Metro board’s safety committee, which is chaired by Downey.

Metro rushed to rearrange train sets beginning  Tuesday night so that no trains would be in the problematic order when the system opened  Wednesday morning.

That required reorganizing about three dozen of the 140 trains Metro operates on a typical day. In addition, safety officers began  Wednesday morning conducting random checks of the system.

Metro operates 184 cars from the 6000 series, and since the cars run in pairs there could theoretically have been as many as 92 trains with these machines in the lead. But because some trains had 6000 series cars in the middle, it’s unlikely there were ever that many trains arranged that way, Stessel said.

It’s unclear right now how many reports of intercom failures Metro received from riders over the years.

“We’ve heard from customers for some time,” Stessel said. “It’s not a daily occurrence, but we have heard reports from customers of attempting to use the intercom and not getting a response. And our engineers have been looking into those issues.”

The transit agency announced the news about intercom failures  Wednesday, the day after Metro General Manager Richard Sarles was informed of the situation and two days after a reported issue involving a Red Line train.

A fight aboard a Red Line train  Monday spilled onto the platform on Woodley Park, and a witness said riders tried to use the intercom to contact the operator but weren’t sure whether it worked.

“The report of an intercom not working on that train would have been caused by this issue on Monday,” Stessel said. The Red Line train  Monday had a pair of 6000 series cars followed by 1000 series cars, he said.

The 6000 series was introduced in 2006, while cars from the 1000 series date back to the late 1970s and cars from the 4000 series arrived in the early 1990s.

Cars from the 6000 series could not receive communications from the older cars because they could not detect that the intercoms were being activated. To fix this, workers need to replace pieces in the 6000 series cars and the 4000 series cars.

Replacing parts on the 6000 cars will allow riders on the 1000 cars to use the intercoms. Metro has  replaced pieces on several of the 6000 series trains and estimates that it should have that work done in early August. It’s unclear how long it will take for the new equipment for the 4000 series to be installed.

Metro has ordered hundreds of new rail cars, and the first set of 7000 series cars is expected to arrive this year. About 300 of these are going to replace the 1000 series cars.