“People were screaming; people were throwing up because the smoke was so thick,” Mr. Kopp said as he stood on a street corner above the 125th Street station. His shirt was covered in dirt from when he climbed off the train into the tunnel.

When the train derailed, Mr. Kopp saw sparks from what looked like an explosion.

“I thought, ‘This is it,’” he said. “I thought, ‘We’re going to burn alive in here.’”