The victim, a 47-year-old San Jose man, was attacked by a group of assailants who were trying to rob him as he waited for a southbound train at about 7:30 p.m. Friday, spokeswoman Christine Dunn said.
Between nine and 15 suspects beat the man, Dunn said. He was left wounded on the ground and was later transported to a hospital.
No weapons were seen during the attack, Dunn said.
The victim's family has requested that his name not be released, Dunn said.
No arrests have been made in the case but transit police are "aggressively" investigating the attack because of the severity of the assault, Dunn said.
"Transit police have been working through the weekend and continue to make progress," Dunn said.
Anyone with information about the attack is encouraged to call Detective Victor Lopez at (650) 622-8048.
