WASHINGTON — The deadly Amtrak derailment near Philadelphia appears to be yet another accident that didn’t have to happen.It could have been avoided if a long-sought safety technology had been installed on its tracks and trains, according to information gathered by accident investigators.On Thursday, Amtrak President Joseph Boardman said the nation’s passenger railroad will finish installing the technology, called positive train control, throughout its busy Northeast Corridor by the end of the year.Seven …