PHILADELPHIA — The Amtrak engineer at the controls of a train that derailed in Philadelphia, killing eight people, remains the focus of an inquiry by federal transportation safety investigators. But a second probe could prove more personally devastating: Prosecutors are trying to determine if he committed a crime.Brandon Bostian, 32, could face criminal charges if law enforcement finds he was reckless or negligent when the train approached a curve at 106 mph, according to former prosecutors. But any prosecution would …