"Dead Freight" 
Season 5, Episode 5

When methylamine was running low, the mysterious Lydia informs Mike that there is train full of it that passes through New Mexico every week on Wednesdays. Great. It's just too bad it would be impossible to get the chemical without killing the train's conductor and alerting authorities that something had happened. Thanks to Jesse's insisting, he, Mike and Walt come up with a plan that would call for bringing the train to a grinding halt, then sucking out the methylamine and replacing the lost weight with water—so it looks like the supplier of the chemical just watered it down, and no one comes looking for the stolen methylamine. Genius.

The plan works, with some bumps here and there, but they get what they need. As the team celebrates with Todd—who was on hand to help—a young boy on a bike appears, and fatefully waves toward them.