Family, friends and classmates of a Naval Academy midshipman who was killed in the Amtrak train derailment packed into a Long Island funeral Thursday to mourn.

Justin Zemser, 20, was traveling from school to his Queens home in the Rockaways Tuesday when the disaster occurred.

“It’s very difficult for me to stand here and talk about my son…he was a friend, a mentor, a teenager and classmate and, of course, our only son,” his devastated dad, Howie, said during a tear-filled service attended by hundreds at Boulevard Riverside Chapel in Hewlett.

“Everything that Justin did he did with affection and a reason behind it. Justin was loving, bright and talented.”

The father said his son, the valedictorian of his high-school class, often volunteered, pitching in to clean up after Hurricane Sandy and working in a soup kitchen.

“Justin accomplished in his 20 short year of life more then most people could do in a lifetime.”