He knows his hobby can become obsessive because it’s happened before. In high school, teachers told him he was capable of getting A’s if only he would stop editing Wikipedia during class. Nowadays, he tries to maintain a better balance. He writes his college course notes by hand so that he will not be tempted by his laptop.

Currently Mr. Ng devotes at least two or three hours a day editing and maintaining transportation articles. He is not alone.

Mr. Ng may be the most prolific member of the WikiProject (as like-minded editors are known) dedicated to New York City public transportation, but there are roughly 20 others. And he is not the group’s only teenage leader.

Shaul Picker is a 17-year-old senior at the Academy of American Studies who edits under the username Kew Gardens 613 — after the Queens neighborhood where he lives. Like Mr. Ng, he began seriously editing Wikipedia in eighth grade. “Some of my edits at the beginning were not constructive at all,” he said. “I’ve become a better writer through Wikipedia, definitely.”

Mr. Picker hopes to become an urban planner, and has spent the past five years improving Wikipedia articles about the subway. He has gathered information from a variety of sources, including listings of old subway timetables he has found on eBay.