I read about this in the New York Times this morning. Good New York story.
Apparently a 3-year-old Queens boy left his mom at a Roosevelt Avenue McDonalds, got on a subway train and rode alone for seven stations until he was tracked down by police.
According to the Times: a detective from the Vice Major Case Team was having lunch at that very same McDonalds when he saw a hysterical woman crying "My son. My son. What happened to my son?" He issued an alert after checking the store's surveillance video. At around the same time, a subway passenger noticed the boy riding solo and told a transit worker who notified the police.
I love the quote from the boy's dad, Jose Lino Marquez, 40: “Everything now is O.K.. My son likes trains,: he told the New York Times.