Photo: Nick Otto / Special To The Chronicle Photo: Nick Otto / Special To The Chronicle

The man on the elevator at San Leandro BART Station wore no shoes and moved in erratic, twitchy motions. Sophia Humphrey, who uses a wheelchair, was nervous enough next to him that she boarded a different car on the train.

The 22-year-old Ohlone College nursing student was going home just before 1 p.m. Tuesday after a morning of running errands and visiting the doctor for a hearing aid check. She was tired, and just beginning to doze off, when something startled her.

It was the man with no shoes again. Authorities later identified him as Jermaine Jeremiah Brim. He entered the train car and spotted a homeless man — who Humphrey said often rides that train — asleep against the side of the car, tennis shoes sticking into the aisle. Suddenly, she said, Brim grabbed the man’s shoes and pulled one off.

Another passenger, since identified as 49-year-old Oliver “Tyrone” Williams, stood up, walked up to Brim and told him to stop. He didn’t like bullies, his family said. Brim dropped the shoe and headed back to the other car, but suddenly turned and headed toward Williams, who was walking back to his seat.

Humphrey heard Brim ask Williams for his shoes. She noticed the light tan leather hiking boots Williams wore had thick laces and would have been difficult to remove.

Humphrey, who is legally deaf but can read lips, was unable to tell what the two men said. But Brim started beating up Williams when he started to sit, throwing punches while Williams put his arms up to defend himself.

Photo: Nick Otto / Special To The Chronicle

Other passengers, including the initial target of the shoe theft, fled the train when it pulled into Hayward Station, but Humphrey and another passenger, whom she described as a mentally unstable homeless man, stayed.

She said she stayed because she wanted to tell police that Williams had not provoked the attack.

Grabbing her phone discreetly, she texted police through an app that helps deaf people communicate, telling them she needed a police response to an assault aboard a moving train.

Brim threw more blows at Williams and wrestled him to the ground, Humphrey said. At some point, a knife emerged. Humphrey didn’t see who pulled it out, but authorities later said it belonged to Williams. She knew then the situation had changed, and could turn fatal. Instinct kicked in. At 91 pounds, she knew she could not intervene without risking her life. But she had to do something.

“I don't know if I had time to be scared,” Humphrey said.

She yelled at Brim to stop, and the other man remaining on the train begged the attacker not to stab the man pinned on the ground: “Please don’t stab him. Hey, buddy, please don’t stab him.” The assailant did not listen.

Brim stabbed Williams in the neck. Then he stabbed him again, and again. A total of five to 10 times, Humphrey estimated. Thick, dark blood pooled on the floor.

She desperately texted police to send an ambulance and to stop the train. It pulled into the South Hayward Station and Humphrey saw Brim flee, strip off his blue hoodie and run down the stairs.

Humphrey dropped her phone and slipped on the plastic gloves she had in her bag. She lifted herself from her chair, lunged onto the floor and wriggled over to Williams. He was gripping the wounds on his neck, but she replaced his hands with her own to apply pressure to the wound.

Photo: Nick Otto / Special To The Chronicle

He was alert and talking, and although she couldn’t hear him, she knew she had to keep him that way. She asked him about the Raiders hat and jersey he wore, about the woman’s name tattooed on his arm. Minutes ticked by. The knees of Humphrey’s leggings became bloodied.

When he moved his neck, more blood gushed out. Where are the paramedics, she thought desperately, wondering whether the blood flow had slowed because of the pressure she was applying or because he had lost so much.

No BART police officers were on the train or in South Hayward Station at the time of the stabbing. Agency officials said Thursday that the first officer was dispatched at 1:02 p.m., that the first Hayward city officers arrived at 1:08 p.m., and that the first BART officers arrived at 1:10 p.m.

Two women who had been in the adjacent train car came in, and one offered her red sweater, which they used as a cushion. Humphrey asked them to call emergency services for an update on paramedics, as it had been “several minutes” since she first called for help.

“I’m not sure exactly the time frame because I was just trying to keep him from dying,” Humphrey said. “I seriously wonder if there had been police officers nearby, or available first aid, there could have been some way to reduce the bleeding faster. Would they have been able to save him?”

With nothing left to do, the women knelt and prayed out loud.

One woman prayed for the bleeding to stop. Humphrey prayed the man in her arms would survive. Finally, she prayed for his salvation.

Paramedics arrived and took over, and Humphrey was ushered away to be interviewed by police, but minutes later, EMS personnel told her Williams was dead.

Later, Humphrey, who is trained in first aid, realized that the experience erased the doubts she had just the day before about becoming a nurse. She knew she could handle herself well in an emergency. She said she was grateful she was there.

“It was a terrifying experience for the victim, so I would much rather myself have to live through something traumatic and be there for someone else, than not be there,” Humphrey said. “This gentleman would have died alone, on the floor of BART, probably before paramedics even arrived. That would have been bad.”

Alameda County prosecutors on Thursday accused Brim, 39, of San Francisco, of murdering Williams, attempting to carjack a motorist outside the station afterward, and other charges. After fleeing the train, he ran to a car dealership where he brawled with a man and tried to steal two cars.

Authorities said Brim, who was facing gun charges in San Francisco, had walked away from a San Leandro hospital and was listed as a missing person at risk before he wandered into the BART system wearing a sweatshirt and no shoes.

Humphrey just wants the Williams family to know one thing: “He was not alone.”

Anna Bauman and Lauren Hernández are San Francisco Chronicle staff writers. Email: anna.bauman@sfchronicle.com, lauren.hernandez@sfchronicle.com Twitter: @abauman2 @LaurenPorFavor