Though Byas has a Bronx address, police described him as a homeless man who apparently had received a summons for trespassing earlier in the night. It was unclear whether he was trying to help the victim, joined in the robbery or inadvertently stumbled into the skirmish, they said.
Both the transit worker, Maurice Parks, and a suspect were hospitalized with stab wounds. A teenage suspect was in custody on a robbery charge and a third was being sought.
Police said the scene unfolded at about midnight as Parks, 39, was leaving a subway station. He later told police that without warning he was knocked to the ground with a blow to the back of the head before bandits jumped him and knifed him in the stomach.
Parks pulled his own knife and fought back. Police said that during the melee, he stabbed two men - one of the attackers and the 27-year-old Byas.
Patrol officers responding to a 911 call arrived to find Parks and Byas - who later died at the hospital - sprawled on the ground.
Parks was in stable condition with a stab wound to the stomach and slashes on his hands. The attacker was in critical condition, also with a stomach wound.
Union officials said Parks, who lives in Harlem, had gotten off work at about 11:20 p.m. after a vacation-relief shift. He became a transit worker in 1997.
