PHILADELPHIA — In the dark blocks of crumbling rowhouses pushed up against vacant factories along Glenwood Avenue, a group of young men hanging out on a stoop on a recent night said it was easy to sneak onto the nearby railroad tracks.

“There’s fences, but a lot of times they are falling down,” said a 16-year-old with long hair and a thin mustache who gave only his first name, Isaac. “A lot of people go down, creepy people, bums — they throw rocks, they throw bottles, but usually it’s no big deal.”

Nevertheless, the possibility that a flying object hit an Amtrak train before it lurched off the rails Tuesday in Philadelphia has unnerved riders and drawn increased public scrutiny to the safety along that stretch of track. Federal and railway officials say being struck by rocks, bricks and even bullets is a longstanding problem for trains in the country’s rail systems. While not speculating on the cause of Tuesday’s accident, a retired Amtrak engineer and a former transportation safety official with the federal government each said that a projectile striking a train would be a dangerous distraction for an engineer.

Investigators have not assigned a cause or blame for the crash of Amtrak’s Northeast Regional Train 188 as it made its way to New York from Washington. Eight passengers were killed and more than 200 people, including members of the crew, were injured.