For Brandon Bostian, the engineer in the fatal Amtrak derailment in Philadelphia this week, a childhood passion for trains turned into a career. Growing up outside Memphis, Mr. Bostian was an “unabashed nerd,” said Lee Allen, his best friend through middle school and high school. “When you heard the name Brandon Bostian, the first thing you would think is trains,” Mr. Allen said on Thursday. “His walls were covered with pictures, he had several model sets. Sometimes we’d just go down to the tracks that ran through town and watch trains and shoot the breeze.” Mr. Bostian was at the...