Transit Police report arresting a Boston man they say took out a knife and threatened to stab another rider on the Ashmont branch in a dispute over the man's mask this morning.
Rafael Perez-Medina, 33, was arrested on a charge of assault with a dangerous weapon following the incident, which began as an argument after Perez-Medina insulted the man for looking "like a duck" after the train rolled out of Ashmont shortly before 9 a.m.
Not long before the train entered Downtown Crossing, "Perez-Medina now armed with a knife (blade fully displayed) stated to the victim 'I'm in the Army I'll kill you,' " police say.
The victim rushed off the train and reported the incident to Transit Police. Officers found Perez-Medina on Temple Place and, after the victim identified him, arrested him, police say.