A man was killed Monday evening after being pushed onto the subway tracks in an unprovoked attack at the 125th Street station along Lexington Avenue in East Harlem, according to the Police Department.

At 6:48 p.m., a man on the uptown platform shoved the person onto the tracks in front of an oncoming No. 4 train, which was unable to stop, a police spokeswoman said. The suspected attacker is in custody, according to the spokeswoman. The police did not identify either the victim or the suspect, but a senior law enforcement official said the man in custody was 24 years old and appeared to have a history of mental illness. He had several arrests in Brooklyn, the first one at age 16, the official said.

Train service at the station had resumed by 9 p.m., but a large number of police officers remained at the scene. Some riders expressed fear of violence on platforms and on trains.

“The subway has been insane lately,” Ray Velez, 60, from the Bronx, said as he waited on the 125th Street platform two hours after the attack. “You have to look everywhere now. It’s just out of control.”

He added, “I wish someone would notice how many mentally ill people we have on the subways and try to get them out of the subway.”

Others who live and work near the station expressed a weary sense of frustration. They described regular encounters with people experiencing problems related to drug addiction, homelessness and mental illness, and their own efforts to remain on guard.