A BART passenger fatally stabbed an 18-year-old woman in the neck and wounded her older sister as they rode a train in Oakland late Sunday in what officials said appeared to be a “random” attack by a stranger.

The double stabbing happened about 9:45 p.m. at MacArthur Station, which was quickly shut down for the night. The station reopened Monday morning.

Agency spokesman Jim Allison said the second victim was taken to a hospital for treatment, while the suspect — described by witnesses as a heavyset white man in his 20s or 30s with short hair — fled and remained at large Monday.

A motive in the violence was unknown.

Relatives of the victims gathered at the station late Sunday night, hugging and crying after talking to police. Malika Harris identified the woman who died as her sister, 18-year-old Nia Wilson, who had attended Oakland High.

A man who said he witnessed a portion of the train attack said he had his back to the victims when he heard a commotion.

He said he saw one of the victims bleeding from her neck and the suspect “poking” the other woman. He said BART police arrived and began performing CPR on one of the victims.

The man said he didn’t hear any yelling or arguing before the attack.

Ebony Monroe, a cousin of the victims, described Wilson as a “beautiful, sweet person” who loved her cousins, fashion and doing makeup. Monroe said Wilson had a boyfriend who drowned in a lake two years ago, and that she was celebrating his birthday on Sunday.

“She needs her justice … she was just an innocent kid, and she didn’t deserve this.” Monroe said. “And he did this for no reason.”

Monroe said a witness described the suspect as coming “out of nowhere.”

After the stabbing, AC Transit assisted BART in getting riders to their destinations.

Megan Cassidy is a San Francisco Chronicle staff writer. Email: megan.cassidy@sfchronicle.com Twitter: @meganrcassidy