Authorities say some 30 passengers watched as Gerardo Sanchez of the Bronx stabbed Johnson at around 2 a.m. Saturday in an argument over a seat.
Police say the passengers were trapped with the knife-wielding attacker and his victim until the train arrived at the station at Seventh Avenue and 53rd Street.
Police say Sanchez was standing over the bloodied victim when the train doors opened. Johnson was pronounced dead when the train arrived at the station.
Sanchez, who's 37, was arrested on charges of murder and weapons possession. He had not been arraigned on the charges as of Sunday afternoon.