MBTA police arrested a Jamaica Plain man last night for allegedly attacking a 62-year-old woman while he tried to snatch her handbag at the Back Bay Station.Robert Johnson, 49, was found by T cops restrained on the sidewalk on Dartmouth Street by an “unknown male” after Johnson pushed the woman to the ground and hit her in the torso while attempting to snatch her handbag, T police said.Johnson is expected to be arraigned today in Boston Municipal Court on charges of assaulting to rob while unarmed, and assault and battery on an elderly person, T police said.