Davey said at a news conference Thursday afternoon they “do not believe this was a door incident,” however the M.T.A. has been previously dogged by concerns about safety. A Goldman Sachs employee, Daniel Enriquez, was shot and killed on the same Q line in late May. In April, a gunman opened fire on a Brooklyn train during the morning commute, wounding 10 passengers. And in January, Michelle Go was killed after she was pushed onto the tracks at the Times Square station.