An A train emergency brake was activated Tuesday morning just before two train cars derailed and hit the subway tunnel wall, according to newly re-minted MTA Chairman Joe Lhota.

Lhota hosted a brief press conference Tuesday outside of the 125th Street station in Harlem, where the A train derailed shortly before 10:00 a.m. prompting multiple train evacuations and major delays throughout the system. He said investigators will determine how the brake was triggered.

"We are evaluating what caused the brakes to go into emergency," Lhota said.

"Two of the cars derailed and scraped the side of the wall," he added. "Two of the cars went right into the [125th Street] station, so people were able to get out. Some people decided to go out the back way on their own, which we never encourage."

Ronnie Hakim, who has served as the MTA's interim director, added that a preliminary investigation revealed track damage, switch damage, and damage along the concrete subway tunnel wall. The derailed A train had not been removed from the subway tunnel as of 11:30 a.m.

"While it looks like scrapes and bumps, we don't know full extent of train damage until it is removed [from the tunnel]," Lhota said.

Witnesses told Gothamist that one subway car door was torn from its hinges during the derailment. They also reported breathing in smoke, and seeing sparks. Lhota said Tuesday that sparks along the tracks set garbage on fire, prompting a "minor... smoke condition."

The MTA could not confirm the speed that the train was traveling at when it derailed. The operator is currently with investigators, according to Lhota.

While Lhota told reporters that seven people sustained minor injuries during Tuesday's derailment, the FDNY supplied a much higher number. FDNY Commissioner Daniel Nigro told reporters on the scene that at least 34 people sustained non-life threatening injuries—that 17 were evaluated on the scene, and 17 others were taken to Harlem Hospital or New York Presbyterian.

According to Nigro, 200 firefighters and 100 police officers evacuated about 500 people from the subway tracks. It was not immediately clear how many straphangers were on the derailed A train, and how many were on three other trains evacuated as a result of the derailment. Meanwhile, the derailment caused an extreme backlog at the Jay Street A/C station in Downtown Brooklyn.

"The department deployed folks north and south of this station," Nigro told reporters. "They went down onto the tracks in three locations. That's where those trains were stopped."

"It was a very difficult operation, operating below ground with that many people," he added.

Today's derailment is the latest in a string of subway-related woes, including a stalled F train that trapped rush hour riders underground for close to an hour. Many of the issues have been attributed to aging infrastructure.

"We need to figure out what exactly caused [this], but it does not look like a failure on the part of the track itself," Lhota said of today's derailment.

"One of the things that I will do while I'm here is to rebuild the confidence in the MTA," he added.

Governor Andrew Cuomo, who effectively controls the MTA, recently asked to increase the number of state-appointed MTA Board members. While Cuomo already has a plurality of board votes, and sets spending priorities, he has insisted that his control is not complete.

Advocates, meanwhile, have accused Cuomo of attempting to distance himself from the system's failings.

"Regardless of the proximate cause of today's derailment, nothing should cause a train to come off the rails," said John Raskin, director of the Riders Alliance, in a statement. "First an F train traps people underground. Then an A train derails. Incidents like this will keep happening until Governor Cuomo takes charge with a plan to fix a failing transit system. What will it take for the Governor to stop making excuses and start funding public transit?"

Hakim told reporters that it is "too early to tell" how this morning's derailment could impact the evening commute. Check Twitter and MTA.info for the latest updates on service changes and shuttle alternatives on the impacted train lines.

Neither City Hall, nor Governor Cuomo's Office, immediately responded to a request for comment.

Additional reporting by Trevor Kapp.