Smoke, followed by scattered pops that some commuters mistake for fireworks but are really gunshots, then panic as a quick escape is thwarted by the subway car’s locked doors. A passenger pounds on the door of the neighboring carriage, desperate to be let out. As the train pulls into the platform, some scream, a man grips his coffee tightly, seemingly dazed — and others tend to the wounded who lie bleeding on the floor. […]