I wasn't sure how to look it up either, so I did a copy & paste on twooffour's words "creepy kid at locomotive" and put "Back to the Future 3" in front of it on a google search. Turns out he is well known as "creepy kid at the end of Back to the Future III". You can find youtube videos of the scene, but basically while Doc is talking to Jennifer and Marty you can see Doc's son Verne in the background motion with his hand then point to his crotch.