Last updated: May 18, 2015, 12:58 p.m.
Here are the latest things to know about the derailment of Amtrak Train 188.
1. Projectile probed: It remains unclear whether the derailed train was struck by a projectile in the moments before the crash. A conductor told investigators on Friday that she overheard a radio conversation in which Train 188 engineer Brandon Bostian said his train had been hit. But the 32-year-old engineer apparently didn't radio his dispatchers with that report, a National Transportation Safety Board official said Sunday. Two other trains -- a SEPTA Regional Rail train and an Amtrak Acela -- have reported being hit by something around the same time that night. The NTSB says the projectile is one of many angles being investigated, and the FBI will examine the train's windshield.
2. Another lawsuit: Philadelphia personal-injury attorneys announced that another lawsuit is being filed in connection with the crash on behalf of several surviving passengers. The first suit was filed last week by an Amtrak employee.
3. Trains running again: Amtrak service between Philadelphia and New York City resumed Monday morning, as did operations on SEPTA's Trenton Regional Rail Line.
4. Train safety: The Federal Railroad Administration has ordered Amtrak to immediately implement an electronic braking system, called Automatic Train Control, at the Frankford curve. The system, already in place on the southbound tracks in that area, likely would have prevented the derailment. And a new, more sophisticated safety system, called Positive Train Control, is set to be in place by the end of the year, though its installation has been hampered for years by cost and politics.