As the train pulled away from Baltimore-Washington International Airport, Darby looked up to see a 20-year-old Navy midshipman named Justin Zemser making his way down the aisle, clad in his dress whites. Zemser was from Rockaway Beach, Queens, the only child of Howard Zemser, who worked in printing, and Susan Zemser, an assistant supervisor at an insurance company. When Zemser was a child, his uncle Richard used to kid that he was destined to be the first Jewish president. In high school, it stopped seeming like a joke: Zemser was the valedictorian of his class at Channel View School for Research, a star running back on the football squad and a regular volunteer at the local soup kitchen. After being accepted by Columbia, Cornell and Yale, he enrolled in the United States Naval Academy, and he was now a third class — the equivalent of a sophomore. Zemser was returning home from school, where he had been supporting friends participating in the annual Sea Trials, a rite of passage for plebes, as freshmen are known. The evening of May 12, he sent a text message to his uncle, telling him he would see him soon, and another to his mom. “Ma, don’t forget to put out my dinner,” he wrote.

Now Zemser stopped in front of Darby’s seat. “Is anyone sitting here, ma’am?” he asked. Darby shook her head and smiled. While he was situating himself, Zemser bumped Darby’s shoulder and elbow. He apologized and opened his own laptop. And for an hour or so, Darby and Zemser sat alongside each other like that, companionably and quietly.

Before Darby realized it, they were already in Philadelphia.

At around 9:17 p.m., Amtrak 188 went through North Philadelphia Station, sounding its bell once. The quadruple lights of the locomotive cut a ribbon through the night, past warehouses and overgrown lots. A few of the tire shops on East Erie Avenue were still open; plastic bags fluttered in the sagging concertina wire that lined the tracks.

If the Northeast Corridor is the most arduous route on the entire Amtrak network, then the three-mile stretch between North Philadelphia Station and the exit to Frankford Junction is among its most exacting sections. This is only in part because of the tight curve itself. Visibility here is also questionable (engineers say the trackside lighting is unpredictable), and trespassers are common. In the wake of the derailment of Amtrak 188, a new wrought-metal fence went up near Frankford Junction, but at the time, access to the tracks was as simple as lifting a scrap of chain-link fence. Even today, it’s not hard to reach the tracks if you want to; plenty of holes and gaps remain. A sergeant with the Philadelphia Police Department told me that the rail bed is popular with addicts and dealers. “And let me tell you,” he said, “you couldn’t build a fence high enough to keep them all out.”

All that foot traffic creates serious hazards for train engineers, who must worry not only about hitting pedestrians but also about being hit themselves, with debris or other hurled objects — an event railroaders describe as “getting rocked.” Familiar to engineers across the United States, rockings are especially common in the densely populated urban centers of the Northeast Corridor, where trespassers target trains for sport. It is not unusual, engineers say, to have two or three objects hit you over the course of a couplet. “Everyone in the Northeast has stories about it,” Edler told me. “After a while, the only ones that stand out are the instances where it’s really crazy, like a railroad spike used as a harpoon, or something.” At night, a rocked engineer has no time to prepare. He or she might be watching the track or monitoring the dashboard gauges, and then suddenly — with a pistol-shot crack — the heavy windshield is spiderwebbed or even caved in.

On the evening of May 12, at least two trains were struck by objects near Frankford Junction. One was a southbound Acela, which was reportedly hit at around 9:05 p.m. The other was a Septa train headed to Trenton. The Septa engineer, who called in the incident just minutes before 188 derailed, immediately put out a “hot track” warning, indicating to all oncoming trains that there was dangerous activity in the area.